Money makes trades hard to compare. A $400 win on a $10,000 account is a completely different trade from a $400 win on a $100,000 account. R fixes that.
Position size stops distorting your review. You can average R across completely different trades, and you get an honest expectancy: if you average +0.3R per trade, your edge exists. If it is negative, no amount of "I was right about the direction" fixes it.
Look for losses worse than −1R. They mean the stop was moved, skipped, or the size was wrong. Fix those first — they usually account for a disproportionate share of the damage.
Tag every trade with its R in the journal and the review page does the rest.