If there is any question of weather, you can (and should) retain your IFR clearance until on the ground and then cancel. However, there may be good reasons to cancel depending upon circumstances. For example somebody may be following you and cannot be cleared for the approach until you cancel your IFR clearance.

A pilot would "cancel IFR" (Instrument Flight Rules) and transition to VFR (Visual Flight Rules) primarily to increase operational flexibility or to expedite an arrival. When a flight is under IFR, it is under the constant control and separation of Air Traffic Control (ATC), which requires following specific routes and altitudes. If a pilot breaks out of the clouds and has the required visual weather minimums (clear of clouds and sufficient visibility), they may choose to cancel IFR to take a more direct "short-cut" to the airport or to avoid a lengthy instrument approach procedure. This is common at smaller, non-towered airports where canceling IFR allows the pilot to land at their discretion rather than waiting for ATC to clear the "block" of airspace. Canceling IFR also relieves the controller of the responsibility for providing separation, which can be helpful in busy terminal areas if the pilot is confident they can see and avoid other traffic. However, a pilot should never cancel IFR unless they are absolutely certain that the weather will remain VFR all the way to the destination, as once the IFR flight plan is closed, they are no longer guaranteed a protected path through controlled airspace.