rotation- when you place an item and want to rotate it around it moves a set number of degrees instead of just how sensitive your mouse hand is. so if you want to have a wall and every one needs to be at the same angle check the button and it should be very easy to align them all up. you can also write the number in the box at the bottom with the X Y axis to do the same job.
snap to grid- just helps you line up with the feint grid on the map (at least in space, cant remember if ground has a grid of the top of my memory.) so again if you want all your walls in a line, snap to grid will easily line them up
turning these buttons basically give you less control on where to put things but it makes it a lot easier to have it all neat and tidy.
It should be noted that when the translation STG (=snap to grid) button is active you CAN NOT manipulate the Y-axis (=height) freely because is also snaps to the height grid - which happens to be 3.048 in my experience.
So if you use Foundation Building Blocks - turn off STG, search for the right height for your floors, walls etc. and place a SINGLE ONE. Then torn on STG and you can use copy/past to align all your elements along the grid. When you need to insert something with a custom height, turn off STG again.
The rotation STG is mostly used for aligning 45°/90 walls when building fake interiors or buildings from scratch.