The high-end data/material rewards (EFC, Biotech, MEF) require a high-paying mission - generally at least 3 million - to have enough credits to do the exchange. All wing missions do this, but only certain high-ranked cargo haulage, massacre and surface scan missions do this, so depending on the system you may see those showing up lots on the wing missions and not much at all anywhere else. (Or you may be in a system with a ton of Elite surface scans and be able to roll in MEF faster than you can use it)
The lower-end (still up to G4) data/material rewards can show up pretty much anywhere (though data courier missions, unless they're long range, rarely pay enough individually to even get G1 materials). Thermic Alloys, Security Firmware Patch, Refined Focus Crystals, etc.
Got to say I've noticed a significant bias towards commodity rewards (which as far as I can tell are still mostly useless - qwent aside) rather than materials.
Couple of other uses:
1) Tech brokers. Much quicker to run a quick mission for Articulation Motors than to run 100s of LY to a station which sells them
2) Extra money - they exchange the commodities in to the reward at galactic average price, but they (especially Platinum) can be sold on for more than that. (Depending on the economy+state of the station, possibly without even having to launch)
