They don't need to crack any password if you don't encrypt your disk because the contents of that disk will be wide open. Simply put the machine into target disk mode and you can pull anything off of it without using credentials. The only way to prevent that is to use whole disk encryption (in this case that would be Filevault) AND use a firmware password (which makes it a lot order to put the machine into target disk mode). This is very useful for when your notebook gets stolen (with it being a very mobile device this is a rather real risk).What's the benefit of encrypting your disk? It's not like any ordinary person will be able to crack your password.
This is not something you use to prevent the government from snooping on your disk (they have many other resources to do that) but simply to prevent people from accessing your data in case your notebook gets stolen. In case a company is ISO 27001 certified it is actually mandatory to encrypt disks in notebooks for that very reason (risk of theft and thus leaking data). There are other certifications which also require you to do this.
Turning on Filevault will create a special key that is used to encrypt the data. You can save it somewhere you deem safe. I remember there was also the option to save the key with Apple. Only in case of the latter will Apple be able to decrypt it unless they have a backdoor in Filevault (which I doubt since it is using AES256).And I think Apple limits the number of attempts. Let alone decrypt the disk. Unless you have something to hide on the state level? Tax evasion or a huge stock of bitcoin.