Apart from its use as a HOB overflow for sumps, a siphon is used to take water out of your tank during pwc. If you design the siphon properly (ie a large tube connected to a small tube), you can control the water flow so you can clean out the substrate ... this is commonly called a gravel vac. You can find those in your pet stores. Some have adjustable flow valves, which is good for a sand base, as you generally need to hover that over the sand to pick up the MULM, but need a slow enough flow that you don't remove the sand in the process.
A siphon is prob the best method to take water out of a tank, and you do need one for your water changes ... unless you want to buy a water pump for it. <You only need a pump if you are dumping water up hill.> Another option is a "python" or various copy. This is just a fancy siphon that is started by a running tap. It also has limited ability to pull water uphill, but you waste a lot of water doing that.