Sorted index includes the cache key and the affinity key columns. Why?

[Andrey Kornev]

Hello,

Ignite adds the cache key and optionally the affinity key (if defined) as two 
last columns of all user-defined indexes (line 301 in H2TableDescriptor). For 
example, given a user index salary_idx including a single field "salary", the 
actual index is going to look like this:

CREATE INDEX "SALARY_IDX" ON "mycache"."PERSON" ("SALARY" ASC, "_KEY" ASC)

I'm wondering what is the purpose of doing so. Could someone knowledgeable 
(Vladimir O., for example) share their insights?

Thanks
Andrey