Are poster sessions important in mathematics? Does being accepted to present a poster at a SIAM conference add something to my CV when I apply to graduate programs or it won't add much?
I am a master's student.
Poster sessions, like any academic activity with a reviewed publication will add something to your CV. Not as much as a full paper, but positive.
Poster sessions are, however, also valuable since they give you the opportunity to meet people and some of those people might be the ones to review an application later. Beyond that, it gives you experience in presenting your ideas to people, which has its own merits.
Even the creation of the poster is a valuable experience if it forces you to focus your ideas. The benefits may be individually small, but there are several to consider.