Some stoves are designed without a grate and if this is the case then there is no need to use one. If the stove has been designed for use with a grate then you do need to use one.
By having a grate the fire is raised off the floor of the stove allowing much better visibility of the flames which is much cosier In many cases, the inside of a stove is not designed to withstand the full heat of the fire - the grate often raises the fire up off the base of the stove so that this is protected. The sides of the stove below the grate may well not be protected by firebricks as the fire is normally above this level when being burnt on a grate. If this is the case then you can end up damaging your stove if you do not use a grate.
Wood burns best on a bed of ashes so it is best to allow these to build up a little; you do not need to clear these away every day.
Many dedicated wood burning stoves have a non riddling grate - this is a grate with no moving parts and is normally solid-looking in appearance - easily distinguishable from a multifuel grate.
If you have a multifuel stove, and are wanting to burn coal, a multifuel grate is essential. Multifuel grates come in a multitude of different types depending on the make and model of your stove. But whatever the design, the job they do is the same; a grate allows air to come from underneath the fuel and to be drawn up through it, giving a really good burn. This allows the stove to be most efficient, giving out the maximum amount of heat.
Many multifuel grates are 'riddling' grates. This means that the grate has a mechanism which allows it to be moved, or 'riddled', to allow the cinders to fall through the multifuel grate into the ashtray beneath. This clears the ash away which has been blocking the air flow through the coals, allowing air to be drawn once again through the coal.
To increase the life of your multifuel grate it is always a good idea to empty out the ashtray every day. If you allow the hot cinders and ashes to build up against the grate, restricting the airflow, then the grate will get very hot and this will shorten the life of your grate.
If you use a damaged grate to burn coal on you risk the coal falling through the grate into the bottom of the stove or ashpan. Excessive heat here can damage the stove.
Grates do wear out over time, but can be easily replaced and it is advisable to do so.