This lowercases every character in the input. It is the simplest normalisation available and the one most often needed before comparing anything.
Why lowercase before comparing
String comparison is case-sensitive almost everywhere. SEO, Seo and seo are three different values to a deduplication pass, a lookup table or a set operation. Lowercasing collapses them to one.
Where it is applied
Keyword lists before deduplication, URL paths before matching, tag and category names arriving from systems with different conventions.
What it destroys
Proper nouns, acronyms and deliberate internal capitals. Lowercase for comparison; keep the original text for anything a reader will see.
Try it: Convert Text to Lowercase on SeoWolf's Notepad.