There are several reasons to hire a transport service (private car) to get into town from the airport.
1) No schlepping bags.
If you travel with the full allotment of allowed luggage, as I do (but that's another thread), it's too much to wrangle on the Tube or train, or down the street. (I did it when my luggage had straps, not wheels, and I was younger...)
2) Door-to-door service.
It can't be beat. Sit back and enjoy the drive then take your luggage directly into your lodging. See #1.
It's just plain worth the money. The price I most recently paid was 59GBP inbound, 54 GBP return (4 people shared the cost). Put it in your budget and you won't notice the cost, only the convenience. Check with your travel agent, sometimes they can get a better price.
4) No fuss.
Book in advance online, pay in cash on delivery at your lodging. (stop at the airport ATM)
5) For first timers, your driver will be as chatty as you want. Get an introductory tour. Get your "accent translator" engaged.
6) For first timers with luggage, it's a harsh introduction to the Tube to discover how many stairs and escalators you have to negotiate, and how far "3 minutes from the X Tube station" really is.