Scholarly and research posters are typically one-time use items; once the conference or presentation is over, the poster hangs on a wall or is recycled, although some conferences "publish" posters online or abstracts in a professional association journal/supplement. However, these "grey" publications often have value as citable items later in the lifecycle of research. Archiving your poster allows you to:
Although your poster can simply be posted on a lab, department, or personal website in PDF format, for more stable and long-term storage and access, submitting it to an online repository such as the Carolina Digital Repository or F1000Research is a good idea.