With a rotating proxy every connection gets a new address. A sticky session switches that off for a while: every request that carries the same session label leaves from the same address until the time runs out or the connection behind it drops.
Use one for anything a site expects a single visitor to do: signing in, filling a cart, a browser profile, solving a captcha and then submitting the form. On PiratProxies the session is part of the username, like this: USERNAME-country-us-time-600-session-a1b2c3d4. The same label gives the same address; a new label gives a new one.
Related plans on PiratProxies
Read next: Targeting and sessions: every part of the username