I recently had trouble with the same question as you. After all, it's not like you move that much more and you're still throwing the same banana.
I think the main uses are for placement and heavy hits. (These can be considered the same thing some of the time)
If you throw a banana and ur glidetoss passes the person, (Example, you start to their right, glide toss throw the banana, banana trips them and u end up on their left) u can get a free, unstoppable smash. This is good for high percentage kills (Free S-Smash!!!) or just racking up damage.
U can use other forms of glidetossing just to position better. Get closer to the edge, place your bananas to annoy your opponent better meanwhile moving. Especially with forward glidetoss->smash, you'll notice u get a lot bigger hits in.
A lot of the time it's hard to see exactly how much glidetossing is helping you so you tend to give up on it, but don't. Mainly work on forwardglidetoss->S-smash, that will probably help you out the most.