Considering there is literally dozens various database formats that share db extension, there exist many possible db to xls conversions, which can be almost any database to Excel conversions.
Many databases (.db) can be exported as Excel spreadsheets (.xls) mainly for reporting purposes or so you can quickly review some data or use them in presentations.
You can usually find a dedicated Excel export in many database management programs, but even some spreadsheet programs may offer limited support for some database formats.