Loss leader to get you to trade more. The existence of USDC makes it easier to move in and out of crypto positions, and to send crypto to other exchanges without taking on exchange-rate risks. Coinbase's business model is to take a fee whenever you transact. The more transactions you make, the more money they make.

It's similar to Google's "make the web faster" efforts (Chrome, V8, AMP, mod_pagespeed, hosting JQuery & other AJAX libs, developer education). If the web is faster, you visit more pages. If you visit more pages, you search more. If you search more, you click on more ads, which makes Google more money.