It really depends on specific conference policies, but in general, student posters are from students and posters are from more senior academics. By separating them, participants of the conference are made aware which posters and presentations stem from students and thus constitute work from less experienced researchers, so that the level and complexity of that research might(!) not be as high. Many conferences also have best poster/paper awards for student contributions.
Also, often conferences offer reduced attendance fees for students, so when chosing "student poster" instead of "poster", you might have to pay less (assuming you actually are a student and have proof of your status).