A security deposit protects the owner from costs should a tenant damage the property and/or not pay the rent as agreed.
The security deposit is not a legal requirement. However, most property owners want one and therefore have a clause to that effect included in the rental contract. It is often the case that the owner will only let you have access to your rented accommodation after you've arranged the security deposit.
Take out a security deposit loan
If you're having trouble advancing your security deposit, then, for rented homes in the Flemish Region, you may be able to take out an interest-free security deposit loan with the Vlaams Woningfonds (Flemish Housing Fund).
Note that if you do so, you won’t be able to arrange your security
deposit online. The reason for this is that when you apply for a
security deposit loan, you need to provide documentary evidence that
you have opened a security deposit account in your name, which you can
only obtain from your KBC branch.