What is the use of setting Gas (limit) in a transaction?

So I recently learnt two things:

1. Gas is a measure of computation on the Ethereum blockchain, a function with more computations will require more gas
2. When sending in a transaction, it is possible to set the gas limit.

What I do not get is the use of being able to set the gas limit.

Given the gas that will be used is going to be determined by the computation anyways, so increasing the limit will not make the transaction run faster, sooner or anything.

But if you set the gas limit too low, then you specifically set up a situation where your transaction will be reverted, if the limit goes below what the computation needs. (I assume if the computation surpasses the gas limit, the changes will be reverted and you still pay the cost right?)

So I am finding it hard to imagine the scenario where adjusting the gas limit will make sense.