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.

Following up the comment - giving a talk is not just a CV line. For most academic researchers, access to funding to attend a conference depends on being a presenter at that conference. So giving a talk is essentially the ticket to hearing what other people are working on, identifying potential collaborators, and keeping up to date.

Journal papers are not really sufficient, publication tends to be slow (though in some disciplines, initiatives like ArXiv is correcting that) and talking with someone is often better than reading a report to understand what they have done. Personally, I always find at least a few talks at conferences that are interesting and give me new ideas but I probably wouldn't have read the paper because it wouldn't have appeared in any of my searches.

Writing an abstract to a conference by itself will only give you the chance to speak at that conference. As others noted, you can put the conference talk on a CV and access funding to go to the conference because your talk is accepted. Some institutions and even grant agencies would count the number of your talks and conference papers -- each for its own abhorrent reasons.

But, there are many other benefits one can get from giving the talk.

• early career scientists (PhD students and young postdocs) get a chance to present their own research and practice for thesis defenses, future job interviews, etc. One gains a lot of confidence from giving a good (or even decent) talk.
• one learns to present their research subject to different types of audiences, depending on the conference. I've often seen great work flying past the ears of the audience because the speaker was too technical, but I've also seen technical people labeling research as "easy" because the speaker avoided technical details.
• a good talk can be heard by a future employer (especially in academia) and they would sometimes offer the speaker a job in their lab, especially if the speaker mentions that he's looking for a position;
• a talk at a conference can also mean a chance to disseminate good quality research that would otherwise be buried in the pages of journal (even a good one);
• in places where scientists are quite isolated from the international community, conference attendance and talks help break that isolation, and even start collaborations with other researchers;
• I personally like giving talks only to give a chance to researchers I won't otherwise meet to comment on my work and even give some helpful suggestions. It is often that the discussion continues after the session ended or at the dinner table.