The link:
https://www.reddit.com/r/formula1/comments/4e24qt/f1_tyres_for_dummies/
If you have heavy tires there is a big chance you could go a long distance without a pit spot, and when you have soft tires that means it will run out quicker, these cars go 300+ km per hour and that means the tires will be basically bone dry by the end of the race if you do not do a pit spot.
Basically the best one would be to find a way to have just one tire to go all race without any pitspot but that would make you very slow if you have tires like that, a soft tire that can go at least 30+ laps and require only one pit stop would be ideal but that hasn't been figured out yet, so these improvements are all just research for that.