Generally, loan applicants use guarantors if they believe that they will be unable to secure a loan on their own. This might be the case for applicants who have poor credit or no credit history. Individuals can serve as guarantors or can have guarantors for their loans.
The use of a guarantor on a loan application does not ensure that the loan will be approved, because the person who guarantees the loan is considered part of the loan application, so the credit rating of that person must be evaluated along with the loan applicant or applicants.