Some champs are still useful when behind or can easily catch up - others cannot easily catch up, are completely useless, and/or have blown any potential to carry due to being early game champs.
Riven and morde are more awful than many other champs when put behind - way less useful than say galio, malphite, soraka
renekton, darius, kalista, lucian (in some matchups) lose out on their carry potential when put way behind early game because they do not scale to the same extent as other champs and/or are more easily punished for being behind.
Ex: think of playing lucian into cait or jinx who is a bf sword ahead of you in laning phase - you are going to get poked out so hard and either miss tons more xp and cs or die, miss tons of xp, miss tons of cs. Not a pleasant scenario.
There are people who ask/ping/yell/whine/whatever for ganks for different reasons. Why depends on: elo, player. Higher elo or a good player will ping because there is either the opportunity for an easy kill or tower objective if the jungler will gank or... because they have been focused a bunch and NEED immediate help in order to not get put impossibly behind where they will basically be a free kill any time they go to lane - even under tower. A good jungler should be able to look at lanes, hp bars, minion waves, champ movements around the map, etc. and determine where the optimal place(s) to gank is. It might change depending on the time of the game (4 min, 10 min, 20 min, etc.) and the champs being played.