So a company made an "informal" offer to me, today, and said a formal offer would be forthcoming if I accepted. Which kinda begs the question.. why not just make the "official" offer immediately? Doesn't seem like it'd cost anything to make an "official" offer.
For larger companies, an official offer is a formal affair. It consists of putting together some documentation and mailing you a letter. (yes a letter, how quaint.) There are three reasons I can think of to offer unofficially first:
- If you aren't interested, why waste time with the letter.
- It gives both parties the opportunity to negotiate salary and the like BEFORE the official offer is written down.
- To keep you. If they don't say anything and wait a couple days to have the official offer ready, you might take a different job.
And another thing not yet mentioned: many companies (especially large ones) have long stretched out approval processes for new hires and other (major) expenses. These can often take weeks or even months.
So you could get an informal, tentative, offer now as an indication that the team lead or department head you're going to be working under wants you, but he still has to get approval from HR, Finance, regional sub board, and worldwide board.
Getting all that, especially with holidays interfering or other days off can take quite a while (my last job, they told me they had all the required signatures but one in a week, a record time, and that last took another 2 weeks because the person was on a skiing trip and could not be reached), and they don't want you to go accept another job in the meantime.
Some companies have a long period between unofficial offer and official offer. As well as some of the things Jeanne Boyarksy mentioned, more and more companies are doing police and background checks before making official offers. There can be other bureaucratic stuff. Sometimes the hiring manager can tell HR that they want you, but it takes HR a while to process it. In a recent case it was six weeks between unofficial and official offer.
Technically the company can withdraw the offer before it is official so never act on an offer until it is official. But they won't make it official before they are done in case the background checks find something. Only you know the likelihood of a background check turning up something undesirable.