I want to know whether it is good taking short jobs, and how it affects my CV and my career.
The effect of temp work on a career depends heavily on the role and the type of positions as well as their length. It's different for every industry and profile. Very short-term work is usually reserved for positions with less experience required and less glamorous work: admin work, receptionist duties and secretarial and clerical work. Such positions often don't have the same career progression as other office work but as an advantage their experience counts in full on your resume. One downside is that once you establish a history of temp work it can be easy to get pigeon-holed into that type of work by recruiters and you'll want to make an effort to confirm that you're looking for full-time work.
Why do employers offer such short-term employment?
To cover short-term absences and other types of leave. Temp workers with renewable contracts are also much cheaper and easier to replace, though this is a much greater concern in countries with very long notice periods.
How can gaps on the CV be explained to a potential employer?
By grouping all short-term work in one header and summarising it:
Management Assistant (2015-02 - Current)
Company X (2015-02), Company Y (2015-03 - 2015-04), Company Z (2015-04), ...
It'd be best to check with someone at an employment agency in your area to find out what the conventions are. You might give a single date range for instance and drop the dates for individual companies (especially for very short-term assignments). You might list the temp agency as your employer instead.
The one caveat here is that you want to avoid padding out your resume: if you were fully unemployed over the summer, you probably can't list temp work as "May 2015 - Current" but if you've worked in these types of positions for multiple years and been mostly employed during that time it would be fine.
Have someone with experience hiring check your resume to figure out what the best format would be and to avoid misrepresenting yourself.