It is not possible to keep an infant in line of sight at all times, especially if they are sleeping in their own room. Baby monitors are very handy and sometimes essential to parents who are busy elsewhere in the house. Modern houses tend to have insulation and sound proofing, making it difficult to hear if a child is crying or in trouble. It is equally difficult to hear if they are breathing properly, especially if their bedroom door needs to be closed. In most cases, baby monitors are needed from the day the baby is born. It should be used as an aid to proper parenting, not a substitute.
There are many types of baby monitors available. The most basic ones will have an audio link, backed up with lights, for parents who have hearing problems. What you need to look for, in essence, is a monitor that has good transmission and reception, good wireless connectivity all over the house and a belt clip that allows for easy carrying while you are busy. More advanced models and makes have temperature monitors and pacifiers in the form of night lights and built-in lullaby music tracks. These extra features are not essential to the role that a baby monitor needs to fill and can put a drain on the battery power of the unit.
Baby monitors with built-in sensor pads do not have any real advantages for parents. It has been found that the sensor pads do not help prevent cot death and can, in fact, simply cause more anxiety for parents. You should choose the monitor that has the features you really want or think you need, rather than feeling pressured into buying the 'safest' one with the most features.