In replays, You really need a reason to be watching it. If you lost to a certain champ, Watch your lane. Replay every trade 3+ Times and see what you could have done differently to gain the advantage in those trades, or what you could have done differently to counter them. PERFECT Example is
vs
This is truly a skill matchup in midlane.
Zed can get close quarter with yasuo and Ult him as soon as he throws his Tornado. This gives zed a MASSIVE advantage in ulti trades.
Vice Versa.
Yasuo can bait zeds ulti with a Q, E away then W to block his shurikens, then continue to E away from zed until the ult procs. Then Zeds Ulti/W should be down so he should be highly vulnerable.
Little things like this you can watch and learn from in replays.
ALSO if you arent familiar with jungle paths, watch what the highest skilled (either yours or theirs) does. learn where they start, what they do etc.
It's all based on why you feel the need to watch the replays