washing soda crystals help wash away deposits of soap and grease.
If you appear to have a sludgy waste pipe problem, pack the plughole/waste trap and overflow with crystals one night, add just enough hot water to moisten them (not enough to wash them away down the drain).
The next morning add a couple of cups of hot water, this will dissolve most of the remaining crystals and they will flow along to the next restriction in the pipe. Give them as long as you can to work, then agitate with your sink plunger or pump, and wash away with plenty of hot water.
Do this once a week or so.
Once the waste is reasonably clean, you can put a cup of crystals and a cup of hot water down the plug hole about once a week, and it will tend to keep the trap and waste pipe clean.
Washing Soda crystals are not the same as Caustic Soda, which is quite a dangerous chemical and will eat holes in your skin and eyeballs.
Taking off the U-bend or bottle trap, and scraping it out with a teaspoon, is useful and instructive.