There are many reasons for the same. The first and paramount one is that you can get a higher loan amount sanctioned, if you need the same.
For example, if the husband and wife both are working, then a joint loan would enable them to get a higher loan based on income of two persons and not one. Even if the wife is not working and has interest or other income, she can still be a co-applicant for a home loan.
The second and other important thing is the tax benefit. Under section 80C of the Income Tax Act, Rs 1 lakh can be deducted from personal income tax for principal payment on home loan (Rs 1 lakh is overall cap for all deductions under this section). This means that if you are paying principal amount payment of Rs 1 lakh a year, you don't have to worry about putting money in other instruments like PPF, LIC etc.
Also, an amount of Rs. 1.5 lakhs can be deducted on interest payment on home loan, under section 24. So, together both the applicants can avail a maximum tax benefit of Rs 5 lakhs on a home loan, as against Rs 2.5 lakhs, if there was no joint applicant.
It's important to note that the structure should be in such a manner that you get maximum tax benefits.
In case of a joint loan, the husband can apply along with the wife or a father and son could apply. It's important to note that the c-owners of the house can only be co-applicants to the loan.