What's the point of a Mask?

[wifiguy]

Here is my current command which is going to try crack a password that is numbers only, with increments starting from digits to 15 digits. At the end I have this mask of 15 characters ?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d is it even necessary to add this to the end? If so, why? The increment of up to 15 characters is all it needs then why I have to add the mask on the end too? 


hashcat.exe -m 22000 hash.22000 -a 3 --increment --increment-min 8 --increment-max 15 -1 ?d  ?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d?d