The difference is really blatant. When you use imperfective future tense construction that means a prolonged action.

Я буду понимать (всегда, каждую секунду, всю жизнь, весь день etc)

When you use future perfective construction that means a one-time action, one that happens quickly in the future at some time. In other words Future Perfect is for general simple actions

Я пойму (что-нибудь в какое-то время)

We use future perfect more, because we speak about something we will do just for once more often, so there's no need to show the action is a prolonged one. All that matters is information, the information that you will perform an action in the future some time.

However, sometimes we need to show that action is a long one.

Examine the folowing:

Завтра я буду танцевать и веселиться
vs
Завтра я потанцую и повеселюсь

The first one shows us that the person will be dancing and having fun for a good amount of time, like for the whole day, or some hours etc..
The second just gives us the information what the person is gonna do tomorrow, in substance, they're gonna dance and have fun, but the action is a one-time one, when the action takes place the action finishes. In other words the person stops dancing and having fun by the time the action occurs, and just gives us the information that they have danced and had fun.

Another example of using perfective and imperfective future tense is comparing two future actions. The action that will be done by some time is perfective, the action that will be in progress by that time is imperfective.

Examine these:

Когда я приду домой, жена будет готовить ужин - Wheh I've come home, my wife will be cooking a supper.
Я буду спать когда начнется игра - I'll be asleep when the game bigins.

Okay, the last use of the imperfective future tense I'm gonna show you is just an uncompleted action, in other words an action which is in progress.

Examine this:

- Завтра будет отличная вечеринка в 1:00 a.m, не хочешь пойти (the future perfect, because you just ask for the information) туда со мной?
- Я не могу, я буду работать (I still will be working by that time you mentioned)