I assume you mean with Function Points, the so-called Story Points in agile practises.
The main purpose is to size stories, to make it fast and do not waste too much time on getting it on too much detail and on a level.
In these agile practises, you don’t spend much time trying to achieve ‘precise’ or ‘perfect’ hour estimates because this tends to be unrealistic anyway. The goal is to reach relatively accurate estimates that are based on each story’s complexity knowing that this may change slightly as we elaborate each story further - and you accept learning by doing. The more you practise the better your estimates and the more confident the team gets with story points.
Have also a look here:
https://help.rallydev.com/sizing-and-estimates-overview Saving Changes...
