This is one of the most frequently asked questions and the answer to which is not definite. In other words we can say that the answer to this particular question depends upon the use of our computer. There are a minimum of three situations which lead us to leave the computer on for 24 hours. If one wants the computer to be backed up or upgrade then it has to be left on for the whole day. In another case, if a machine is used as a server it has to be on all day. That is to say if the machine works as a print server, file server or web server on the internet then one have to leave the computer on for 24 hours. Again, if one is running anything like SETI@home and one needs to produce as many results as possible, the computer is to be left on.
If one does not fall in these categories then he has a choice of either putting it on whole day or keeping it off. The first and foremost reason for turning off a computer is being economic. A PC consumes about 300 watts and if a computer is kept on without any work then it is wastage of energy. This can be minimised by turning off the hard disk and the monitor when it is not in use. However, turning the computer on and off stresses the components of the computer. An example can be cited to prove this. When a CPU chip runs it becomes quite hot and when the machine is turned off it cools down. This expansion and contraction may have some effect on the joints that hold the chip in place and also on the chip itself. If this is the case than TV should have similar problem as it also contains almost all the components of a computer. However the TV is never on for the whole day.
In conclusion, we can say that it completely depends upon the user whether he wants to keep the computer on or off.