I think that you are asking, why use OUs with GPOs when I can use security groups? I can think of a few reasons:
1) Clarity. If you create a logical OU structure (e.g. create "computer" and "user" base OUs, create "server" and "PC" child OUs under "computer", create "administrators" and "accounting" OUs under PC, etc.) and link GPOs to their appropriate OU without changing the security filter, you will be able to understand at a glance what GPOs are affecting what user or computer groups. If you use security group filtering only, you'll need to examine each GPO's security filter or use some other time-consuming method to see what is going on. If you only have a few GPOs and you are the only person managing the GPOs it may not really matter, but as the number of GPOs and the number of people managing GPOs increases the security group filtering only method becomes very confusing very quickly.
2) Efficiency and manageability. GPOs linked to higher level OUs apply to all subordinate OUs. Through a well designed OU structure, you can effectively apply group policies with pinpoint precision and minimal redundancy. For example, more generic computer policies that apply to both computers and servers should be linked to the computer OU (e.g. generic IE/Edge settings, generic security certificate deployment, powershell environment settings, etc.), policies to apply specifically to PCs (e.g. client only Windows firewall rules, MS Office related rules, client software deployment rules) should be linked to the PC OU, and custom rules should be linked at lower level OUs (e.g. specific file share mapping, special permissions, etc.). If all your GPOs are linked to the same OU, you'll need to remember every time to change the GPO priority order so that more important/specific policies take precedence over more generic policies (this is extremely easy to forget). I think you find that troubleshooting "GPOs not applying" problems becomes much easier if you have a logical OU structure with minimized security group filtering. (NOTE: some group policies, like the group policy loopback policy, can behave in unexpected ways if you use security group filtering).
3) Speed. GPOs that use security group filtering take slightly longer to process. GPOs that use WMI filtering take much longer. It may not matter with only a few GPOs, but when your GPOs start numbering in the hundreds you'll need to think about optimization. If you want to minimize GPO processing times OU-based GPO assignment is the way to go.