Of course, you will need Conditional Access to secure your Office 365 tenant. There are many articles written about it and why you need it. I am not going to do this.
So what am I going to do? I will give an overview of the, in my opinion, most commonly used Conditional Access rules for SMB.
When to use them? When you have a customer working with Azure Ad joined / Compliant Devices and you really want to secure access to your Office 365 tenant.