The town hall is the most important building in your village.
All kinds of buildings generates from it.
Level 10 town hall is the highest level attainable in clash of clans.
Upgrading your town hall to level 10 means that you can have an access to all kinds of troops and buildings and can upgrade everything to its maximum level.
It means that all your troops can attain its highest level possible that there is.
You can have the level 5 dragon which is a very strong troop for air attacks.
You can also have access to all the different kinds of spells that there is and can upgrade the spell factory to its maximum level which will give you a total of 11 housing space for spells in total enabling you to bring more spells to war.
When it comes to defensive buildings, you can build stronger ones like the inferno tower making your village even more difficult to attack.