To put this very simple: The Java edition is the real
original Minecraft, it's the version as it was initially developed by Notch. And yeah, there are some differences.. Fair warning: I never bothered with the Win10 edition myself so I can't fully comment on it, only hear-say.
The main difference though is customization. On the Bedrock / Win10 edition you can't easily change stuff. I don't know for sure how you'd update your skin, but mods and such are a no no. With that I'm referring to (for example) stuff like this:

This is Forge + Industrial Craft 2, Applied Energetics, Tinkers Construct, Integrated Dynamics + Tunnels and my trusty resource pack :) That green / purple block thing you see before me? It's an auto-crafter, it can actually craft stuff on my command. Those blue / cyan screens in the back ground? They give me access to my "electronic" storage and automated crafting recipes.
That blue pipe attached to the chest? It's collecting items from it and transports it onto "digital" storage. How items get there? Well.. That row of white blocks? Those are machines; one drills for ores, the other refines / grinds it, the next one smelts all the dust and that gets collected in the chest.
The Java edition provides you with pure freedom. You won't find skin packs from famous movies to purchase simply because the Minecraft community makes those for themselves and most of us share with said community. Check out the PMC
skin section for example.
And said Java edition allows you to upload said skin directly from the launcher:

Heck... On PMC you don't even have to bother with uploading using the launcher (I had to because I have a rather personalized & custom skin): on PMC you just click "Change my skin" whenever you see something you like and done.
At the risk of sounding like a marketing drone: did you know that PMC has almost 2
million site submissions featuring Minecraft stuff? No, that's not 2 million skins, but I'm sure there's enough stuff here (skins, projects, mods) to keep you busy for a long time :)
And that really is the power of the Java edition. Full customization, full access, full control over your game. Even running your own server can begin right from the launcher you see above. You can't say the same for Bedrock / Win10.