1. Cats will grow bigger from time to time. The litter tray needs to be changed to fit their size.
2. After months and years, scratches on plastic tray will capture germs and dirt that is hard to be cleaned or wash away. For the health and hygiene of your cat/s, it's best to change the litter tray (especially the one made of plastic).
Besides the ordinary litter tray, there are various kind of cat litter tray available in the market: self-cleaning litter tray; board made litter tray; covered litter tray & etc.
Board Made Litter Tray
It's made of box-type hard board. Normally it is only meant for 1-2 time use sage. It is normally made by recycle materials and it is easily decomposed.
Covered Litter Tray
This is referring to litter tray that has covered on top. The purpose of adding the cover is to keep the odor within the tray. This manages to reduce unpleasant smell in the house. It also provides more privacy to cat (most cats don't like to be distracted while toileting).
Self-Cleaning Litter Tray
The title may give you an impression that this may be an electronic automated kind of cat litter tray. To be more precious, I would rather use the word 'semi-auto' to describe these trays.
Basically it's not much different from the normal covered cat little tray. What makes it different is instead of scooping cat poo, cat owner just need to pull the 'handle' and let the feces drop in a plastic bag, pack it and throw it. I have one of this too (Smart Sift), it really helps to make cleaning litter tray easier. The only disadvantage is: the space is too small for adult cats. I have now kept this away due to Kusky doesn't like to use the toilet any more.
Once cramped litter has dropped to the bottom, pull out the second level and remove litter with the plastic bag. As easy as ABC!