Azure Marketplace should be used by default unless you have specific concerns or organizational requirements. Some common examples include;
Complex software setup that requires an application to be included as part of the base image.
Installation and setup of an application could take many hours, which aren't an efficient use of compute time to be added on an Azure Marketplace image.
Developers and testers require access to a virtual machine quickly, and want to minimize the setup time of a new virtual machine.
Compliance or regulatory conditions (for example, security policies) that must be in place for all machines.
Using custom images shouldn't be considered lightly. They introduce extra complexity, as you now must manage VHD files for those underlying base images. You also need to routinely patch those base images with software updates. These updates include new operating system (OS) updates, and any updates or configuration changes needed for the software package itself.