She ain't just setting him up in a cosy cottage with a garden and care staff - she's giving him access to top secret UNIT history! He's getting all the dirt on the Doctor, Liz, Jo, Benton etc from the 70s/80s! He's getting to see all the alien artifacts collected over the years! Kate is going round there for tea and a chat every Sunday, sharing stories her dad used to tell her and updating him on what the Doctor is up to and Wilf's eyes are sparkling every time while trying not to cry because he can't have these chats with his granddaughter.
Spearhead in Space is really funny because Liz spends her first scenes with the Brigadier so clearly believing she's stuck playing the Scully to this man's Mulder. "Oooookay, so you're a UFO-chasing conspiracy nut who's somehow got government pull" NOPE. The Brigadier is one of the most thoroughly regular people on Earth. He just happens to have discovered that his world gets invaded by aliens sometimes.
(And really, the fact that he responded to that discovery by going through government channels and getting an official Earth-defense organization set up with U.N. backing and oversight tells you a lot.)
young kate stewart when her father brings home a guy who he exclusively calls 'the doctor' (claims he has no other name), spends half of his time arguing with, yet spends a solid three whole weeks mourning when he disappears each time, and spends most of his time with her telling stories about him: