HSR1234 there is absolutely noting to stop you using the arrivals channel directly on disembarking. However there can be a number of reasons for using the transit channels.
1) you do not wish to take all your carry-ons with you into Doha.
2) you have prohibited items, such as alcohol, which need to be left inside the airport secure zone during transit
3) you wish to use the lounge facilities before leaving the airport, etc. etc.
Going through the transit route you will be able to use the lounges to deposit your hand luggage in the secure lockers in Al Mourjan and so on.
You will find much more information using the QR Forum dashboard widget
About-DOH-airport .
UK Passport holders no longer require a Visa to enter Qatar.