The Helipad is likely used as a place of business by the Los Santos branch of Higgins Helitours as evidenced by a company sign at the entrance to the helipad and their name on tha hangar buildings.
Helicopters purchased from websites will spawn at the helipad and if the player lands a stolen helicopter it will also continue to be available here.
Michael's helicopters are stored in helipad 1, while Franklin's ones are stored in helipad 2.
After the The San Andreas Flight School Update (update 1.16), players will have access to the helipad available for free, as the Swift will be available for them from the beginning of the game. (PS3/Xbox 360 only).