There are some functions in Office 365 that are easiest to do as a global admin, rather than as a delegated admin – primarily when you need to connect to a tenancy with PowerShell, and when you need to add new services to a tenancy where you have delegated admin access.
Here’s an example of attempting to purchase services as a delegated admin:
You can see that as an global admin, you can add new services:
Thankfully, Microsoft makes it possible to designate an account as a global admin without assigning it any licenses for Office 365. This account cannot consume services (no email, Lync or SharePoint) – Unfortunately, this account cannot be used to administer SharePoint without being given a SharePoint license. However, it’s still a great idea to have an account to use for administering your tenant (or your clients’ tenants) without being forced into purchasing another license.
Another good idea would be to use this account as the first account set up in Office 365 – this account will become the tenant admin, and using that non-licensed admin account keeps it separate from any users. This way, you don’t end up needing to change your tenant admin if the user you created first leaves the company (especially as that account cannot be deleted, only renamed).