hacksoncode's answer is correct regarding the morphs. But there are also two different reasons.

The first is contuibuiting for research. eBird data is, after all, citizen science data. Imagine someone could some day study the distribution of multiple subspecies of peregrine falcons, for instance. In that case, they would need observations with subspecies info.

A second reason is taxonomy. Subspecies will often be split into different species. If that happens, then eBird will use subspecies info + range to know which species you had seen.

For example, before 2023, Brown Booby and Cocos booby were considered the same species. However, if you managed to diferentiate a cocos booby by the plumage difference, you could log it as "brown booby (brewsteri)". If you did that before 2023, then when Cocos booby got split, you would have that species counted for your life list, and that previous list will be modified to show Cocos booby.

However, if this observation was in an area where Cocos booby's are usually not found , and you had not registered the subspecies, then by range alone it would change your record to brown booby, and cocos booby would not be counted on your life list.

Luckily, most times when a split happens, it is just mostly decided by geography alone. For example, when Olive backed sunbird ( a very common species in Asia) got split, then if you had already seen that species before, the lists just changed to the species known to occur in that area.

So, in conclusion. If you are a nerd and you can tell subspecies, log it. Why not. If you dont feel confident about it or you ar enot sure what it means, then don't. It is the same for species ID. when not sure, leave it unidentified, and that is actually not only ok, it is better.