In 1993, after Cheers ended, Frasier and Lilith (Bebe Neuwirth) divorce offscreen, and Lilith is awarded custody of their son Frederick, with Frasier granted visiting rights. In the pilot "The Good Son", Frasier explains that he left Boston because he felt that his life and career had grown stagnant (and he had been publicly humiliated after climbing onto a ledge and nearly committing suicide before being talked down). Therefore, he returned to his original hometown of Seattle, where his father Martin (John Mahoney) and brother Niles (David Hyde Pierce) live, to begin a fresh start.  Frasier works for the radio station, KACL, as the host of his psychotherapeutic radio show, The Dr. Frasier Crane Show, produced by his producer and friend, Roz Doyle (Peri Gilpin), who has many ex-boyfriends. Later, his father Martin, a retired Seattle Police Department detective who was shot in the line of duty, ends up moving in with him. Frasier becomes concerned about his father in his current state as he can barely walk, and requires a cane to move. In Cheers, Frasier says that his father is dead, and that he was a scientist. He also says that he is an only child. This inconsistency is later explained in "The Show Where Sam Shows Up": At Frasier's apartment, Sam Malone tells Martin and Niles what Frasier had said about them, and Frasier explains that he was trying to distance himself from his family at the time. In Cheers, Frasier tells bar patrons that he is an orphan. He confirms in "To All the Girls I've Loved Before" (1988) that his mother Hester, portrayed by Nancy Marchand in "Diane Meets Mom" (1984) and then by Rita Wilson in flashbacks in "Mamma Mia" (1999) and "Don Juan in Hell: Part 2" (2001), is dead off-screen.  Frasier hires a live-in physical therapist, Daphne Moon (Jane Leeves), to look after Martin. Daphne is an eccentric, working class Englishwoman who professes to be "a bit psychic". Moreover, Martin brings his beloved Jack Russell Terrier, Eddie, whom Frasier despises. After some initial apprehension, Frasier grows very close to his new family.  In "IQ" (season six, episode 19), he is revealed to have been an Oxford University alumnus besides being a Harvard alumnus. In the same episode, he is said to have scored 129 in a school IQ test (his brother Niles scored 156).

Answer this question "Why does he hire a live in  physical therapist?" by extracting the answer from the text above.
to look after Martin.