Whether you need liners depends on the material of the planter you are using.
You do not need liners for marine-grade aluminum and fiberglass planters. Neither of these materials degrades from constant exposure to water and soil. Additionally, marine-grade aluminum and fiberglass planters can be made watertight, further removing the need for a liner.
Liners are good to use if the planter material will degrade when in constant contact with soil and water, such as a wood planter. Also, a liner is good to use in interior applications if the planter is not watertight.