Just a few things off the top of my head, i'm sure some pro installers out there have even more uses for it.
1. Signal Generator (this unit makes a true sine wave unlike some of the old units that made a stepped square wave)
2. Ported or bandpass box tuning - It will tell you exactly where your port is tuned
3. Measuring the T/S parameters of speakers (Fs, Qts, Vas, ect) can be measured and calculated with some knarly math. We will provide a spreadsheet or formulas to ease the pain.
4. Test passive crossovers
5. Find the Fs of any speaker (got some nice tweeters you want to bi-amp but not sure how low you can cross them? This tool will tell you)
I'm sure there are other uses as well