Important to have. Even more Important to know how to use correctly and safely!
If you buy a throwline the first thing you must do before going anywhere near the water is throw it. Check the knot attaching the rope to the bag is strong and secure
repack the rope ensuring there are no Knots, no tangles so it will be ready for flawless use when you do need it.
Unsure on the correct length? Always better to have more than less. If your after a 15m line, get a 20m line. That way you have spare if you need to payout or need a bit more reach.