What can you get from submitting an abstract?
You submit an abstract to a conference if you would like to give a talk there. The abstract is then used to determine whether or not you get that chance (based on how interesting/relevant/sound the abstract is judged for that particular conference). So for the conference, this lets them filter (a bit) the talks that get presented there, for the author, this is the prerequisite for being allowed to give a talk.

Giving a talk at a conference is a means for a researcher to disseminate their research ideas and to advertise themselves. Discussions following a talk can lead to future collaborations, and giving the right talk at the right time can lead to eg a postdoc position.

"Talks given" is also a typical category on a CV, so even beyond the immediate talk, the fact that you gave the talk can be beneficial in competing for jobs.