Test tube racks are commonly used in laboratories to keep test tubes upright so that the equipment does not roll away, spill or become accidentally cracked. Test tubes are delicate equipment, usually made of glass.
Using glass has many advantages in a scientific environment, as glass can be sterilized, is easily cleaned, allows for easy viewing of the tube's contents, is non-reactive to many common chemical reagents and conducts heat evenly. However, glass is also very prone to breaking under sudden shocks, such as a drop off the lab table. In addition, test tubes are completely round and lack handles. Test tubes often do not even have flat bottoms, so it is impossible to rest them upon a table top as you would a cup. The roundness is to prevent mass loss when transferring materials from one container to another, but it means that a test tube laid down on a table is prone to rolling. These features make test tube racks a vital part of any decently outfitted lab's equipment.
.A test tube rack keeps tubes in place when they are not being actively used. It also allow for easy organization during experiments. All of the samples from one particular place or containing one particular substance can be placed on the same rack. Finally, test tube racks allow for good maintenance of the equipment. A test tube that is always returned to its rack is less likely to develop chips or cracks.