Guitar is personal Preparation item of Miku.
While its in inventory, resting at Home will grant +1 Reason as well as increase Doom by additional +1%, on top of usual increases from Resting.
This will produce a message in the log: Your loud guitar practice can be heard in the whole building.
Synergy[]
The increase in DOOM helps get stat bonuses from Miku's Mayhem mechanic a little bit earlier, on top of obvious Reason increase.