You can always use it in the garden to start seedlings....

I use one for seasonal displays. Right now it covers seashells.

Here it is from last Christmas.

Klseiverd, sounds like what you have is the dome from an anniversary clock. They're harder to find than you might imagine w/o buying the clock also. Anyway, I have a space in an antique mall, and usually use them for the various seasons. I like to put mine on a clear glass plate to get the full effect of what's in it. I usually glue an old glass(cheap!!)candlestick to the plate to give height~that's what the dome will go on.

As for what you put inside~for the summer months I put seashells, with sea glass. Summer was a bird nest(doesn't have to be a real one!) Fall, is a few autumn color flowers w/glittery twigs. Winter is Christmas-y, so i've used a vintage style glitter snowman, setting on a 'bed' of glitter(snow). You could use Christmas ornaments, maybe the collectible kind, or a little Christmas scene. Use your imagination and have furn!

"Doesn't have a knob on top, but sure it's not just a glass dome to put over something for display"

Just curious why you are sure it's not a dome to put over something to display? Glass cloches usually have some sort of knob or handle to lift them up and set them down. Not to have a handle or knob usually indicates it's to be set down and left there. Is the glass thin and delicate or heavy and thick? Garden cloches are rather dense and heavy of course, or they'd be smashed directly just by using them. The only other option than display dome would be a specialty item it would be hard to guess at. For instance, I have a similar glass 'dome', blown to hold a pillar candle when inverted, and nestled in a wrought iron cradle suspended from a chain. The glass is very thin and delicate. My guess, unless you have a better description of it, is also some sort of display dome.

Domes of that shape with a knob on top can be found in physics labs. Those are used as vacuum chambers. They sit on a soft seal and as the air is evacuated, atmospheric pressure presses the chamber into the seal. (Also found enclosing anniversary clocks.)

These also have been used in labortories to created a controlled enviroment. The rim is coated with a sealant grease and when the dome is seated on a matching seal surface. It creates an enclosed space in which a gas mixtures or controlled humidity may be admitted. Sometimes, this is used to protect objects from atmopsheric oxygen to prevent further aging. The space is filled with dry nitrogen to purge oxygen and moisture. Object thus stored can be held for months without further oxidation.