If your needle is not fully sealing in the seat [worn needle and seat], the gas from the tank will flow past it and into the cylinder, past the rings and into the crankcase. It will dilute your oil and ruin its lubrication properties---it must be drained and replaced.
You can use the fuel shutoff as your engine off switch [starving the engine of fuel] and stop the fuel leaking past the seat but that's a pain.
I recently replaced the needle and seat on a Walbro carburetor but it would've been cheaper to buy a Chinese knockoff carb...