Simply because you'll be getting more wine for your money.
If you think about a bottle of wine, there are many costs involved before you buy it... The wine itself (growing the grapes, making the wine), and then there's the bottling, label design, transportation, storage, marketing, distribution and taxes, plus a load of other costs depending on what the business model for that wine is.
The part of the process that you, the consumer, appreciates most is the actual wine, the stuff in the bottle. So how much wine are you paying for? Here are some examples:
- Spend £5 on a bottle and you are getting 20p worth of wine
- Spend £7.50 and you are getting £1.66 worth of wine
- Spend £10 and you are getting £3.13 worth of wine
- Spend £15 for £6.04 worth of wine
Shocking isn't it?
This will of course vary depending on various factors including where the wine has come from (transportation or import taxes), to who is distributing the wine (direct will have lower fees than through the big wine & spirit companies, any middle men will add to the cost), to who sells the wine (you'll get more for your money from a wine merchant than from a big supermarket chain), but the figures can be used as a guide.
Thanks to @ozclarke and @Condor_Wines and @robersonwine for sharing.