I am thinking of wrapping up a 7-wt fly rod for streamers to use for trout in larger rivers (streamers or other big bugs), and maybe some steelhead or light salt. What are the key benefits of a 10-ft. over a 9-ft. rod?

Depends on your lines and terminal gear you use, 10 ft rods are more stress on the shoulder over many days of fishing and heavier sinktips that some including me while steelhead fishing only makes it worse. The upside is mending is easier. In my golden years I can say I've preferred my 9 ft rods overall.

The longer the rod the better the leverage is for the fish, not the fisherman!.

While not in my golden years, I’ll second Spencer’s opinion.

I personally prefer 10 ft , made up a couple 2pc 10ft fly rods for steelhead about 4 yrs ago, my 9 footers will probably never see sunlight again.

Fishing is not a matter of life or death, it's much more important than that

I had this same dilemma a few months back. I was building a 7wt. for the same purposes as you (streamers and occasional steelhead fishing). I ended up splitting the difference and building a 9' 6", and I'm happy I didn't go 10'. Even at 9.5' it's taxing on the shoulder to throw streamers all day (I'm throwing them on a sink tip). Ultimately I'd build for your main use case, 9 ft. if it's mainly going to be a streamer rod, or 10 ft. if mostly for steelheading. Or you can split the difference like I did.

Which blank did you use for the 7-wt?

I used a CTS Affinity MX that I ordered through Herb Ladenheim who you can find on this message board. It's a "moderate-fast" blank (where I know most people recommend fast or x-fast for streamers) , but I've had zero problem throwing a double streamer rig on a sink tip with it.

Maybe not applicable here, but I find 10' rods better for fishing the beach as it gives you just a little more ease in your back cast against the slope.
That said, depending on how tall you are or how far you have to wade, they extra length will also keep your line out of the water on a back cast.
I tend to do a lot of single hand spey casts, or if I have a fighting but use the second hand too. I am guessing that the longer rod would facilitate this. This is a great way to get heavy flies and sinking tips out of the water and back to the fish.