A very good question. Generally, a product/ web analyst first identifies the website elements and events they need to track in GA. H/she creates and documents the events schema to ensure high analytics data quality before sharing with the developer in Excel/ Sheets. The developer now goes through the tracking setup process of writing datalayer codes, creating variables, tags, triggers and QA in GTM and GA.
After a decade of GA implementations, we have figured out there are scores of such event tracking needs that can be fairly automated on top of GTM right from standard codes to tags and triggers creation with 100% accuracy. An analyst can install these predefined tracking templates on their own with zero or limited developer dependency and save hours of effort and money. They don’t even need to login to GTM. In short, Tagmate is automation of your GTM setup, but not an alternative.