In all honesty if your saddle properly fits your horse you should not need a 5 -point breastplate. I truly believe they are a bad design - look at any picture of an event horse jumping with one on and see how the shoulder is blocked. Use a 3 point/hunting breastplate and all should be well. Btw ever see Michael Jung or WFP wearing one? Never.
It's used for extra stability for your saddle when riding over larger fences, cross country, etc. The extra point attached to the girth helps alleviate some pressure so the straps aren't pulling solely on the withers, since normal Y-style breastplates will usually just attach to the D-rings on your saddle.