A good training partner is almost essential if you want to perform to your own maximum level. At home you can two boat test and tune. Before a race you can meet on the start line each punch a lay line, meet at the top mark and know which corner is favoured (if you have three boats send one up the middle), turn around and do the same down wind. At a regatta you can debreif what happened in the previous race and make sure you both learn as much as possible about what happened in order to do better in the next one.

Some people get very protective and don't like "sharing" with someone else but when push comes to shove there is a guy (or girl) at you club who you race closely with each week and sharing a few tid bits probably won't change that and the odds are when you turn up to a big regatta you'll probably still be close together. If you don't work together you may be battling for 14th and 15th. Pool your resourses and your respective positions might not change but you might be battling for 4th and 5th.

Examples:

49ers at the olympics. Training partners 1st and 2nd.
Moth worlds. Training partners 1st and 2nd.