Why so many people get into the stackexchange groove and participate is a fascinating mystery to me. I think the motives are quite diverse. Of course we are all made of molecules and so the question of why anybody does anything could be considered on-topic here, but let's not.
If people want a quick feel-good moment by helping, or if they subconsciously need a quick look-how-smart-I-am moment because they suffer in a look-how-stupid-you-are workplace, it doesn't matter as long as the outcome is positive.
But there's the rub. What's positive? If we look back to stackexchange, it's good answers to good questions. If the intent behind leaving a compressed answer as comment is to in some way help in the process of answer-generation, it might be a good idea to check up to see if the practice is actually helping or not, and doing something like that is hard if you already are commenting because you don't have enough time to answer.
The question I've asked; What is the overall positive result of leaving compressed answers as comments? might cause one to wonder if there is a metric for positive result. There isn't an obvious one.
Does this actually help attract good answers? Does it intimidate people from trying to write an answer? Does it discourage people from leaving poor answers or a poor answer getting accepted? I think some of each happens and it varies on a case-by-case basis both because of variations in the question and variations in exactly which people view the question pre- and post- compressed answer as comment.
There aren't built-in tools to look at this - in fact it's an excellent long-term research project for someone interested in AI and data-mining.
So right now, the answer is nobody knows!
That puts it in the realm of meta, where ideally an eye is kept on things like this. @Jan just mentioned this has been bumped, an example of eye-keeping-on perhaps.