Most simply, small, inviting fences help get horses going and confident while leading them down a road in their training to where they can answer a more complicated question. In buying a small set of portables and building a small schooling course for yourself, you need to select fences that are adjustable so that you can build up as your horse develops.
A great way to build “inventory” is to use simple things like existing logs, hay bales, and other materials. These make great fillers and small, simple fences for young horses. To progress being prepared to compete in a recognized competition it’s nice to have some schooling fences that look more like what you will encounter at an event, like a roll top and a table.
A small roll top is infinitely useful; on flat ground it is straightforward and inviting, but at the top of a hill it is a whole different question, and at the bottom of the hill it’s another question still. Later you can use two small roll tops together, or in combination with a ditch or a bank.
Another good fence would be a simple table, or any fence with top spread, which is a different shape from a rolltop, much as a vertical is different from an oxer in the show jumping ring. Horses jump differently over fences with a spread.
Keep in mind that while they are cheap and useful, logs will eventually rot and you will have to replace them. They are a good jump material to start with, but eventually replacing them with jumps constructed of pressure treated lumber will give you a schooling course that withstands the elements and will provide you with years of schooling opportunities.