Any withdrawal will always get a brokers attention.
First off, they will be concerned that you might not be happy with their service, so they will often ask your account manager to follow up with you to see if there is something wrong before they process your withdrawal. It's not intended to delay your withdrawal, in fact, you will find some banks do it as well.
Secondly, some payment processors(Neteller) charge hefty fees, which the broker covers. They obviously expect a to get that money back by way of trading activity(commission + spread + swaps). If you were to immediately withdraw, they will investigate and may even pass the fees on to you.
The next is money laundering - the brokers have legal obligations and when you deposit and withdraw without any trading activity, that will trigger a red flag and your activity will be investgated.
Brokers have no interest in witholding your withdrawals - it just makes clients unhappy, but inapproproate withdrawal activity does trigger red flags that need to be addressed.