Normally, there are 2 scenarios when you need to add an IP in the Netblocks section on your domain:

1. If you have an internal IP that is sending emails on behalf of your domain, you will need to add it in your Netblocks and make sure the emails sent by that IP, are being sent in SPF alignment for your domain.

2. If you are using just 2 IPs from a certain service for example, it is advisable that you only add those 2 specific IPs in the Netblocks section, instead of adding the entire service in your Enabled Senders list - the reason being that you do not want to add extra IPs from that service that should not be authorized.

Related article:

What are Internal Sources?