Here's my take as a current full-time RA based on advice from grad students & faculty, but I'm open to being corrected by others on the forum:
1) Better rec letters
2) Are you really that confident that theory is the right path for you? There are tons of grad students that switch from theory to empirics all the time. Full-time RA roles will give you better a taste of what serious empirical research looks like before ruling that out completely.
3) Gives you a pretty good sense of what the environment is like at grad school and whether you'll like it. My building has tons of grad students & full-time RA's that interact with each other, debate on research ideas, work with faculty PI's, etc.
4) Fellow full-time RA's in your team & in other teams will be your future colleagues/coauthors in the profession. Sure you're a theorist, but wouldn't it be valuable for another coauthor to demonstrate your theory with empirics?
5) Theory and empirics aren't two completely separate things. Don Davis suggests "If you want to write applied theory, read empirics." The argument is that theoretical work is only interesting if it has some relevance in the real world while making contributions to theory. Working as a full-time RA helps you get better intuition on what other empirical researchers think are relevant/important questions, allowing you to better cater your research to a wider audience.