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

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

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

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