Why doesn't a femto cell help my cell coverage?

Episode 1726 (38:13)

Max from Boca Raton, FL
Femto Cell Example

Max uses an Obihai Voice Over IP device to make phone calls using his Google Voice and the internet. Leo says he also has a T-Mobile FemtoCell and it routes his calls through to the closest cell tower to him. Leo says that doesn't really help if your tower isn't a good one or you have terrible coverage in your area.