Banks look at your earnings and expenses to assess if and if so, then how high of a loan you can afford. Usually, this happens as you provide additional information yourself (e.g. the amount of your income and your rent) and send or upload this (e.g. your pay slips).
Your salary account also contains information on your income and expenses. Through one look at your account, this information is automatically recorded without you having to manually enter anything. If you apply for a loan at your main bank, they simply look at your account movements so that you don't have to collect all the documents yourself.
Through the new European PSD2 payment service directive the interface to all banks is standardised in a way that you allow digital access to your account through a secure login to your bank account. We are able to automatically fill in all necessary data regarding your income and expenses for you - just like your main bank.