If we didn't use =, the tooltip and its value would be dominantly appended. This would work OK if neither tooltip nor its value had occurred earlier in the relevant tiddler list, or if tooltip had already been set to precisely same value earlier in the tiddler list. However, if one of them had occurred and not the other, then one of the lists would end up longer than the other, breaking the correspondence between the names and values lists. For example, suppose we start with:

$names="to class"
$values="[[Tiddlers]] [[red]]"

Now if we add a tooltip parameter with the text Tiddlers without using =, we'll get the broken result:

$names="to class tooltip"
$values="[[red]] [[Tiddlers]]"

...because [[Tiddlers]] was dominantly appended, yoinking it out of the position where it was needed for the to attribute. Using =, no deduplication is performed and the elements being added are simply tacked on to the end.