Scholarly and research posters are typically one-time use items; once the conference or presentation is over, the poster is hung on a wall somewhere or recycled. However, these "grey" publications often have value as citable items later in the lifecycle of research. Archiving your poster allows you to:
Archiving your poster can be as simple as posting it online in PDF format to your lab or departmental website. However, for more stable and long-term storage and access, submitting it to an online library such as the Carolina Digital Repository or F1000 Posters is a good idea.