Currently, the TemplateData editor uses commas to separate parameter aliases. But the comma is a valid character to use in the parameter (alias) name, and this causes problems.
An example can be found by the link: https://ru.wikipedia.org/wiki/Шаблон:Трамвай/doc.
When you editing any parameter with an alias that contains a comma, this alias will be split into two.
For this reason, we need to use a delimiter character that will definitely not be used in the parameter name. There are quite a few such characters, and the most logical of them for separating values is the pipe. It is also used to split aliases on Wikidata.