Flag poling is when a person\student who is already in Canada leaves and returns without crossing into the USA, in order to renew their study\ work permit in some cases their PR. Students are choosing it as it is a quicker process.
It all depends on the applicant. The postal service requires you to send all your documents and you don’t have to be physically present to answer any questions. Your permts get rstored on the basis of your documents but it is just a time cosuming process Where as in the flagpoling option your permits are restored by answering a few questions to the officer checking your documents and is less time consuming. The wait period is shorter though if the applicant is hesitant is anwering questions then they should go for the postal service option.