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Why is lord Ganesh worshipped first before any puja?

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

I've also seen people worshipping lord Ganesh before they start off the marriage precedings.

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?