5 ft is on the short side, despite what every calculator out there will tell you. I had 5, then bought 8 ft lines, and still think I could use a bit more. Learn from my mistake, and buy 10 ft. You can always cut them down to 8 later, but if you buy short you'll have to buy again, and attach to your shanks again (which in my case involves removing the tower again)... a PITA.
fluid passing through a narrow passage loses pressure to the walls (I don't know the full fluid dynamics for this). Longer lines cause more loss of pressure to the walls (roughly in linear proportion to the length). The idea is that you want to have 12-15psi of pressure on the keg (to carbonate), but a very low pressure on the tap (so liquid flows slowly and doesn't foam up). So you use a long stretch of tubing to lower the pressure that reaches the tap.