This as actually an incentive very common in some areas, in others it's discouraged. (generally speaking the more progressive an area is in regards to business the more likely you are to see it)
How does it work?
It's actually extremely simple and in principal how salary is supposed to work.
Effectively your assignment hits your lap on Monday and it's due Friday for example. On Friday I don't care if you spend 20 hours or 80 hours if you deliver on your deadline something of at least the expected quality I'm happy. (albeit if the hours are WAY off we probably need to adjust expectations to at least be closer to reality)
That means you can take whatever days or time off you like so long as you deliver and you're there at times we need to organize or plan.
(And yes there are companies who are very successful who operate in exactly this fashion, I've been known to force my staff to go home when they start putting in too many hours because burn out is ultimately more costly then minor delays)
Is it a good thing?
I'm sure there are valid pros and cons for this structure, but based on my experience and what research I've found it's nearly universally true getting a few hours less than forty a week winds up more productive than any over.
What? How? The key here is the longer hours you work, the less productive you become. While by no means exact I go by for every hour you work your productivity drops about 10%. This is for two reasons, first you start producing less in general, second as you're mentally taxed you're more prone to mistakes that will require time to correct, eventually you hit a point of doing more damage than benefit. (Again this is NOT an exact science, individuals vary, etc)
In addition you need detox time to get things together before you get back to work or it's like starting with several hours already put in. My office we don't keep strict hours. (no time clocks or anything) we have a few meetings you need to be at and I try to enforce the unofficial no more than ten hours a per 24 hours policy.
If it's so good why is it uncommon?
The first is simple, there are countless people who disagree with the research for various reasons. If you disagree then frankly you see any time they are not working as lost revenue.
The second comes from lousy corporate policy. There are people who's job is more based around attendance than accomplishing a set task. (think support staff like you computer techs, etc) Often for the sake of "fairness" or to have some all encompassing policy companies will make all "full time" employees work "at least" 40 hours.
Culture is another huge thing here. Some culture you live to work that's your pride and joy. Not working long hours is not live up to your full potential, to be a lazy bum. There are also problems in business culture where your "dedication" to the job is the primary factor in which advancement and raises are concerned. Often companies make the poor choice of seeing "Dedication" as "How long do they slave away at their desk" anyone who isn't working long hours is clearly not very committed to these people. (this mentality is both flawed and undermines productivity)
The last cause is self inflicted. There are tons of reasons people work long hours. Perhaps you're struggling with a difficult problem, trying to meet a deadline that you were overzealous about, feel a need to "step up" or that you're under performing, etc.
Typically these are lame reasons that are you treating a symptom and not a problem. The problem is the overzealous deadline, that you're under performing, or that your company culture is based on time put in, not on actual merit, etc.
By all means from time to time we all need to put in a long day or two, but those should be the rare exceptions, not the norm. The norm should be you get your work done in a nice cadence, it's of excellent quality, you go home. (generally if you land under 30 hours you're not being ambitious enough on your deadlines, if your over 45 you're being too ambitious) Otherwise I'm go to judge you based on what your cadence, the quality of your work, etc.
Where do I find such jobs?
General the more progressive a place is the more likely you'll see these sort of jobs, more traditional places tend to actively resist these sorts of things.
For example, while roles like this do exist in Florida they are extremely rare and looked negatively on by other employers as "unprofessional". It's a right to work state that in most avenues is extremely conservative, as such companies here for the most part work in the same manner they have for centuries.
Meanwhile if you look at states like Oregon while this type of company isn't the majority companies with "unlimited PTO" (one name for this style of company) isn't uncommon. It's viewed neither as professional or unprofessional, just a way of doing business. Many of these companies experiment with a variety of practices that were only started in the last decade often shedding a number of the traditional ways of doing things.
Ultimately companies all over are slowly adopting more progressive policies like this, but it is EXTREMELY slow. It's also primarily happening in markets with a shortage of quality workers like software development.