Here are some research abstracts to point you in the right direction of what are the proposed benefits:
“Pre-exhaustion (PreEx) training is advocated on the principle that immediately preceding a compound exercise with an isolation exercise can target stronger muscles to pre-exhaust them to obtain greater adaptations in strength and size. However, research considering PreEx training method is limited.”
“In conclusion, PreEx training offers no greater benefit to performing the same exercises with rest between them compared with exercises performed in an order that prioritises compound movements.”
Couldn’t really find much research on it, probably cause my pubmed skills suck :p. Anyhow, hopefully that helps. I’m thinking pre-exhausting is big in the bodybuilding world? First time I’ve heard of this method or maybe just forgot about it.