The power you have been given can never be taken away. Even if you kill your patron and destroy the contract, your power remains.
But you need a living patron for a lot of things.
1) There are items for which you must have a living patron.
For example Pact of the Tome. You have to get that book from someone, you can't make it yourself.
2) Some of the warlock's more powerful abilities require assistance from their patron.
For example, you can ask him to restore your spells.
3) Patrons are often powerful beings with a vast store of knowledge.
It's not bad to have a direct line to someone who has more knowledge than 100 libraries put together and can give you an immediate answer.
4) Patrons sometimes give gifts to their favorites.
Sometimes they are magic items, sometimes more power.
Whether it's to gain allegiance or as a reward for performing a deed.
BUT
Depending on the wording of the contract or the verbal wording of the pact, the patron has some power over you.
There is a certain connection between you and you may be "strongly motivated" to take certain actions.
In certain ways, you give up a part of your freedom.
If it is written in the contract that:
"""""If it is required and does not exceed a reasonable risk, the recipient of the power is obliged to destroy the enemies of his patron."""""
If you refuse, you will have to face the consequences, which are also usually stated in the contract.
The usual punishment is usually something like an unbearable headache or something similar.
But even the Patrons can't force you into a suicide mission (Go kill the king).
After all, what good would power be to you if you were to die in a few days during a battle that you have no chance of winning.
(You'd have to be a VERY stupid Warlock to sign a contract that says they can demand anything from you without limits.)
So if they want something beyond your contract, they'll usually offer you something as a bribe.
For example, gold, magical item, more power, or changing some of the rules of your contract.
So a Warlock without a patron/contract is partially weakened, as he loses the ability to use/obtain certain powers and items. But he is certainly not helpless.
A warlock with a patron/contract cannot be forced to do anything, but can face nasty penalties if he goes against the contract.
But sugar is better than a whip for devils, and Warlocs are always power-hungry.
So if you really don't want to do something you should, they can motivate you to make you want to.
The dream of every Warlock is a contract that gives you power and the necessary assistance of a patron and does not require anything from you in return.
If one of the Fey Lords really likes you, then it is possible to obtain such a contract.
In the case of devils it is also possible, but there you usually pay with your soul once you die.
