I've seen that hindus worship lord Ganesha before starting any puja *(Act of worship)* or important tasks.

I've also seen people worshipping lord Ganesha before they start off with marriage precedings, and on marriage invitation cards. 

They also keep idol of lord Ganesha in their cars, home entrances etc...

So why they worship lord Ganesha always before they are going to start off with some good/important tasks?