אַרְבָּעָה מַרְאוֹת הָאֵלּוּ מִצְטָרְפִין זֶה עִם זֶה ... לִפְטֹר אֶת הָעוֹמֵד בְּסוֹף שָׁבוּעַ שֵׁנִי.
These four appearances [of Negaim] combine, one with the other ... to exempt that which remains [unchanged] at the end of the second week.
How do the four nega'im colours combine to exempt a tzara'at-sufferer? Even if there were no rule of combination, once the tzara'at has remained on his skin for two weeks, he is released from his impurity.
As an example, let's say one had a snow-coloured nega'. At the end of two weeks, half of it has turned plaster-coloured. Without the rule of 'combination to exempt' he would still have been declared pure. What cases is this rule applying to that we would not already know?
(I note that Bartenura ad loc. brings up a case of the entire nega' turning from one colour to another. I don't understand how that is a case of applying the 'combination' rule, but maybe that's one avenue to explore to arrive at an answer.)