I'm not entirely certain of whether this is providing information that the OP doesn't already have. Apologies if this is a repeat of known knowledge.
In the early evolution of the horn as an orchestral instrument, it had no keys, so the player could only play notes in the harmonic series of the instruments, plus-or-minus hand-stopping (which changes the pitch of a given instrument by a fixed amount) and whatever amount of "bending" the player could achieve with their lips. In higher ranges the harmonic series gets close enough together that the player can actually manage the full scale, but at lower ranges that's impossible and in the mid-range it makes certain notes or runs very difficult to play quickly or with exact tuning (and impossible to get both!)
Instead of keys, the horn came with a set of crooks, which the player could substitute in (at the cost of a few 10s of seconds) to change the key of the entire instrument.
You couldn't do this in the middle of a piece but you could certainly do it between concerts, pieces, or movements.
So if, as composer, you want horn notes that aren't compatible with a single pitching, within one piece. Then you are obliged to get multiple horn players each with a different crook, and thus in a different key.
So, ultimately: "to be able to give each group the better matching parts concerning intonation?" is correct, but with the emphasis on "it might not actually be realistically POSSIBLE" rather than "it might not sound as nice".
Nowadays the concept is of course completely obsolete, but the remnants of the instrument's history remains. (And it gives horn players an excuse for why they were playing the wrong notes: "Oh, oops, I forgot it was transposed" :D )