A staking plan from my perspective does a number of functions.

1) It's a form of record keeping of what you bet and what happened.
2) It's a form of balance keeping so you know how well you fare. I use a spreadsheet which helps to do all the little things like correct calculation of profit etc, so you can watch out for the important matters.
3) It's takes part in the decision making process of whether to go ahead or not. (if say you win well and achieved a target.)
4) It can help to work out your bet sizes. That's providing you are balancing it with likely outcomes (which you have a good picture about), and prices.

Because betting on horses is a limited end game and we are collecting from a 85% return pool, it's important get value and that is all about the ratio of Benefit / Risk. If the benefit outweights the risk then it's in your interest to move forward on that using a staking plan. But if the benefit isn't there in the first place no staking plan is going to be effective for you, unless you want to get rid of your money...

With that there are a couple of sources which I have found both informative and helpful.

http://www.form-pro.com.au/Q&A.html
(this is a page with very sound ideas on how to bet)

http://www.practicalpunting.com.au/staking/
(this is a download page for a free article on staking)