Having a lot of papers published in unimportant conferences VS very few in good ones?
People do things for a variety of reasons. Even writing a paper for smaller workshops has some benefits for everyone (from a PHD student to a professor) 


Good reason for the affiliation to cover expenses for the suggested trip. Usually CS workshops are co-located with important conferences and even a workshop paper may cover your expenses (not every time if you continuously abuse the system) to actually watch the entire conference. 
Workshops have limited attendance but may still be organized by reputable professors / scientists close to your area of interest. So, they are very good for networking. In a major conference, it is easier to get "lost" inside the many participants. 
Practise makes better. If you write 5 papers (even when some of them were for a workshop) writing your 6th paper is going to be easier, instead of trying to write your "seminal" paper. 
Reviewers have the "strange" habit of sometimes rejecting your paper. In this case, sending your paper to a smaller workshop (after rejection in 1-2 major conferences) where it gets accepted, lessens the sense of rejection and still "patents" your results on which you can expand later. 
Workshops sometimes have a "best paper award" which may even lead to journal publication, when the same paper might not had a chance in a bigger conference. In workshops the competition is smaller (usually 10-15 accepted papers), so you have something like 5-8% chance for something like that.
Sometimes when you work on a specific project you  might discover something that although is not good enough for a major conference or later expansion, is still a compact solid idea that may help others. So, a good workshop paper may disseminate this idea to a larger audience.
Unfortunately, although quality should beat quantity, this is not the usual case. In some research projects, grants applications it is better to state that "A is the author of more than 50 scientific papers with an h-index of ... " than "B is the author of 2 papers". Same when you look at the Google Scholar / DBLP record of an author. It is better to see 50 papers (which among them are 10 really seminal papers) instead of just 10 perfect papers that usually leave a gap in the author's bibliography (blank years).


In this sense, even workshop papers serve their purpose if you treat them as professionally as the rest of your papers (they are well written and still scientifically solid and correct).