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What is the use of percent signs (%) at the end of lines?

I know that % is commonly used to uncomment parts of the code, but recently I read that there is another use for the percentage sign but I don't remember what it was neither where I read it.

For example in the following code % was placed at the end of several lines. What does % do in this case? (If required, I will provide a MWE.)

\newif\ifintoc
\def\@schapter#1{%
\if@twocolumn
\if@at@twocolumn