The purpose of vaccination is to prevent serious illnesses, which are usually fatal.
By administering the pathogen (usually a virus or bacteria) in a weakened form, the body will make antibodies against it and it is protected against the disease in question for a certain period of time.
An immunization always contains several subgroups of a pathogen and usually contains several pathogens at the same time. Therefore it is a very strong and unnatural stimulation of the immune system. As a result, animals can have a significant relapse if they are in the process of healing from another disease (for example an allergy). Allergic or toxic reactions can also occur as a result of the excipients contained in the vaccine.
However, vaccinating is often necessary because protection against certain diseases is necessary or because it is mandatory in a number of situations. For example if you want to take the dog or cat with you on holiday abroad.