A number of garden crops require some kind of support in order to grow properly: peas, climbing beans for example.
Tomato cages are also a form of trellis. 
Other garden crops don’t necessarily need to be trellised, however a trellis can be used as it maximizes garden space and helps to avoid many of the insect or disease pests that can infest the plants when they’re in contact with the soil. Some examples include cucumbers, squash and tomatoes.
There is an almost unlimited variety of trellis options. First of all, homemade v. commercial trellis.
Commercially produced trellises are quick and easy, are generally more structurally sound. These can be purchased at garden centers, home building centers or online.
Homemade trellises are generally more economical and can be constructed to fit the specific needs of your crop/bed.
Trellises are not only useful for vining or climbing type plants such as peas, beans and tomatoes, plants such as cucumbers or squash which tend to prefer to grow horizontally but do best when not in contact with the soil.
Keep in mind that trellises that are re-used from previous years can also help transmit diseases. As such, they should be treated to kill any residual bacteria or spores prior to returning them to the garden.
