I used to have treatment plans. They involved a list of things I wanted to work on. They also involved a time estimate of around how long it was going to take for me to be showing some progress with those things.

I think that the official reason for treatment plans is 'accountability'. That has two translations:

Firstly, 'accountability' in the sense that since you are paying for a service and all you are entitled to some kind of estimation as to how long it is going to take (and by extension how much this is going to cost). You should know whether your therapist thinks you can make some solid gains in 6 weeks or 6 months or 6 years... You should also be entitled to work on what it is that you want to work on. For example, if you want a fear of flying treated so you can take a plane to get to a conference in one years time then you aren't getting the service you paid for if they want to talk to you about repressed childhood memories and the meaning of life.

Secondly, 'accountability' in the sense that since insurance companies (or government agencies) are paying for a service and all they are entitled to some kind of estimation as to how long it is going to take (and by extension how much this is going to cost). They like to see progress and little measures of progress. Some clients quite like this too. My DBT therapist joked about turning my daily mood ratings into a graph. She very nearly had to alter the treatment plan to include 'homicidal ideation' (just kidding).

;-)

If the therapy is more long term... If the expected gains are such things as feeling more connected to people and being more creative and feeling happier and the like then it is harder to do a treatment plan and it is harder to estimate when real gains are likely to occur. Most therapists want some idea of what it is that you think you need help with but different orientation (and different therapists) have a different notion as to whether they are going to 'attack' symptoms for fast gains or whether they are going to get to know you over time for more general nebulous hard to quantify kinds of changes...