If 1/2 PEX would have been large enough to do the runs but the contractor used 3/4 instead, then:
- he (you?) paid the higher per-foot cost on the larger tube and paid for each adapter
- the tube holds more water, so if this is a hot water run, it'll take comparatively longer for the hot water to arrive at the fixture.
If the runs really do merit the use of 3/4 PEX, then:
- there's a bit of a flow restriction at that undersize manifold which results in more pressure loss than you might otherwise have had, but if this is in an average home, it probably won't be noticeable.
In the world of plumbing there are endless permutations of fittings and size combinations that could be made, and a great many that actually are available. Sometimes exactly the right purpose-made fitting isn't used (in other words, an "odd" combination of adapters/sizes are substituted as on your job) because the plumber decides to get it done with whatever he actually has on the truck today.