Female cats will nearly always be routinely neutered. It is of course possible to let a female have a litter and then carry this out afterwards. However it should be remembered that it is not always possible to find homes for the kittens and there are often unwanted kittens and cats that need good homes. If female cats were left un-neutered they would continue having two litters a year for life.
Male cats should also be routinely neutered to prevent their fighting and wandering tendencies which inevitably lead to either early death or injury by road accidents or from diseases contracted from their life style. Urine marking with its strong odour is also avoided.
Bitches are usually neutered to avoid the continued problem of their seasons which occur twice yearly and also to avoid 'false pregnancy' which often occurs 6-8 weeks after the season. Early neutering will significantly reduce the chances of mammary tumours later in life and of course avoid the occurrence of infection of the uterus (pyometra).
Male dogs are most often neutered to avoid behavioural problems such as aggression, wandering and urine marking.
Please do not hesitate to ask for advice on this subject if you have any concerns. The operations involved (castration and hysterectomy) require a general anaesthetic. Hysterectomy is of course a more major surgery and although occasional problems can be encountered the procedure is usually routinely straightforward.