Data arrives glued together more often than it arrives clean. A comma-separated blob in a single cell, a pipe-delimited string from a log, a semicolon list pasted out of a form field — all of it needs breaking into one item per line before anything else can process it.
What the split does
You specify the delimiter and every occurrence becomes a line break. One long string becomes a proper list.
Why this is the first step so often
Almost every other list operation — sorting, deduplicating, filtering, counting — assumes one item per line. Splitting is the pass that makes the data eligible for all of them.
The reverse exists too
When the processed list needs to go back into a single field, joining it back into a delimited string is the same operation in reverse.
Try it: Split a Delimited String into Lines on SeoWolf's Notepad.