There are a vast number of things called "spiritual practices". They are found in all the different paths and schools, and are done for different purposes and effects. A person could spend his or her entire life seeking them, learning about them, trying them out; and end up with nothing but confusion and very little gain. On the other hand, there are many instances where a person was given or followed only one practice, or a few, and reached spiritual realization or enlightenment.
The most important point to remember here, is that it is the "right" practice, at the right time, in the right way that is appropriate for a person. Unfortunately, that is generally not what that person does or is given to do.
First, take the situation in which someone gives a practice for someone else to do. In most cases, the giver of the practice is tied to dogma and personal qualities that limit his or her awareness of what the recipient really needs. That "teacher" does not have the realization or understanding necessary to dispense the practice. However, that does not stop the person from doing so. That leaves the recipient, most of the time, with something that is either ineffective, partially effective, or detrimental.
Next, take the situation in which a person self-prescribes a practice. In most cases it will be something the individual is "drawn to". This generally means that the person has an affinity or like for the particular practice. That also generally results in doing something for the wrong reason(s) with results that are not beneficial.
Here we could take the example of 2 individuals. One of them is very sensitive and is drawn to things of beauty and refinement. The other is very coarse and likes rough, harsh things. For example our first person might like chamber music and art museums, while our second person might prefer wrestling and stock car racing. If each of these people was presented with a choice of 2 things to do, each of which was called a "practice" we would expect that they would choose differently. For example, if the two so-called practices were 1. to go to the ballet, or 2. to play football in the mud, we would usually find the first person drawn to #1, while the second would prefer #2. That would result in the strengthening of the qualities that each now has.
But in much of spiritual teaching, it is just those qualities which must be balanced before the individual can go much further. Thus the role of the teacher is to look at the current state of the student and the goal, and to determine a way to get there. This will generally be done in a step by step manner - with small (or larger) revelations interspersed (with the exception of some sort of huge divine revelation and transformation).
The teacher, or guide, needs to provide that most important thing for the student; and that is what the person needs to make the next step. Thus, using our example above, the teacher might also suggest those two activities as practices, but applied to the other persons. In this way they could start to gain a degree of balance.
This doing things, acting "silly" or obnoxiously, taking on the qualities of the student as a mirror, making statements, assigning work, giving practices, and many many more approaches, all can be considered as methods to assist to students to become more aware of what they are doing or thinking, how they are approaching life, tasting what the next step is, and then making it. All these approaches are part of what a teacher does to help the seeker. In the course of this process, there is almost always a swinging back of forth of the pendulum from one corrective and its excess to the other side. Eventually, the swing of the pendulum becomes less and less, and finally comes to the center. This is the process, and it is most effectively applied by someone with the knowing to do so, rather that the individual to himself or herself.
If we consider the story of the person who sets out to cross the ocean without having done it before, in an unfamiliar boat, and with limited and obscure direction, we can start to approach the situation that is existent in most self-application of practices and spiritual learning. That problem is also most of the time compounded with the additional difficulty that the limited instruction or information that the sailor has comes from a so called "teacher" has not crossed that ocean either. Because of that, we can see that it is more likely for the sailors not to get to their goal than reach it. And we can also see that the better and easier way would be for our sailor/seeker to follow the guidance of the mariner who has made the trip before and who knows the waters well.