Apologies in advance for the dullard questions - I have already put the dunce cap on...

I'm about to change the tyres and my understanding is that my syncro originally came with 14" wheels...
1. Why would they be replaced with 15" wheels?
My plans this year involve two long runs to France (Approx 4-5000 miles in total), daily commute (9 miles there and back) and weekend jaunts (usually to the Welsh coast - 50 miles or so). There will be no hard core off-roading. So....
2. Why should I not go back to 14" rims?
3. If I keep the 15" rims what size road tyres should I put on them? Likewise 14" rims?
Really grateful for any thoughts. Also aware that I ask a lot of questions and provide very few solutions - sorry!