On a side of a brush or something else holding the texture, you can see the other edge of the texture you are using, which looks incredibly bad in game.

Here's the thumbnail of this guide so you can look at it closer:
To the left is the thing everyone, including me, usually wants to fix, and to the right is the fixed variant.

For floor, wall, and other such big textures, and (maybe) only works with "LightmappedGeneric" (it enables filtering back again when the texture doesn't take up it's full space on screen, so creates less moire patterns)

• Open your .vmt file, check that it has "LightmappedGeneric" at the top again, and if it is, add flag "$pointsamplemagfilter" 1
  
• Save, should work now!

For SMALL textures (that don't loop a lot like floor or wall textures, fits for more like for a bed texture or a chest, and on semi-transparent and transparent textures this way is mandatory!!)

• Download VTFEdit from here, if you still haven't;
  
• Either import an image to be your texture with any import settings desired, or open a .vtf file you don't want to be smoothed;
  
• Under the "Image" tab to the left, check "Point Sample" flag.