Why do quarters and dimes have ridges and pennies and nickels don't?

Answer 1

The ridges on the edges of coins are called "reeding". When coins were made of precious metals like silver and gold, reeds were used to prevent theft by shaving, i.e. using a sharp instrument to remove a tiny bit of metal from the edges of a coin. A shaved coin would very slightly smaller but not enough to be noticed. If a crook shaved enough coins they could make a nice profit selling the scraps. Mints adopted reeded edges so any attempt at shaving would be immediately obvious.

Nickels and cents have smooth edges because those denominations were never made of precious metals. When the US switched from silver coinage to copper-nickel clad coins in the 1960s reeded edges were no longer necessary but the Mint decided to continue using them so the new coins were as much like the old ones as possible. However new designs and denominations like the current Presidential dollars don't have to match any previous coins so they're made without reeding.

Other countries now use specialized reeding to make it easier for visually-limited people to tell coins apart by touch. For example some denominations of euro coins are similar in size but have very different edges with, for example, wide reeds, narrow reeds, or "interrupted" reeds consisting of alternating ridged and smooth sections.