As long as you stick to software appproved by Apple and aren't prone to poking around in lots of unsavoury places on the internet, then iMacs ans Macbooks are pretty safe without a third party internet security system - I certainly don't use one.
Security of any kind is only necessary in relation to how and where it is going to be tested.
If you send a daily e-mail to your aged granny, you probably don't need any security at all, if you spend twenty hours a day surfing the dark web, then you need something that is the cyber equivalent of a high wall with broken glass on it, and large savage dog on the other side of it.