"Endothermic" animals are more commonly known as "warm-blooded." The term is a bit of a misnomer because "cold-blooded" animals also have warm blood; however, they are "ectothermic." The key difference lies each group's ability, or lack of ability, to regulate their body temperature. Being endothermic offers several advantages, as well as disadvantages.
Ability to Live in Cold Areas
Most ectothermic reptiles are native to warm and tropical areas. The reason is simple -- they regulate their body temperature based on the sun, limiting them primarily to areas that have a high amount of sunlight. They seek the sun to warm up and seek the shade when their bodies get too warm. Most mammals on the other hand, internally regulate and maintain a constant temperature, allowing them to live in colder regions where the sun is less available to warm their bodies.
Most cold-blooded animals don't internally house their offspring, but rather keep them in locally placed eggs. This can be especially detrimental if the environment is full of predators. Endotherms, on the other hand, maintain warmth inside their body, providing a nourishing environment and live births.
Natural Bacteria Protection
The human body's natural antibodies and defences are relatively automatic. When infected with a sickness or bacteria, humans' internal temperature naturally rises in an effort to kill off the germs. Ectotherms, such as lizards, don't have this luxury and must instead bask in the sun -- assuming it's available -- for longer periods of time. This makes ectothermic regulation slightly costlier in terms of risk as compared to endothermic regulation.
One huge downside to endothermic regulation is that it's energetically costly. The bodies of mammals are constantly maintaining temperature through metabolic processes so constant food intake is critical. Very large mammals, such as elephants and whales, spend the majority of their day eating, and the bulk of their caloric intake goes toward maintaining their body temperature.