A clinical trial has a fairly limited number of participants, which can restrict the number of contexts a drug is used in. Due to the smaller number of individuals taking part in the trial, some very rare side effects may not become apparent. During market use, these very rare side effects are likely to arise, and so post-market release surveillance is used to capture this data to confirm the safety profile of the drug is within accepted levels.
Clinical trials only study a drug for a limited period of time, which may restrict long-term side effects from being observed. This is the same with long-term use. Post-market release surveillance also allows data on these effects to be obtained.