Mostly it will. If it doesn't see if you can find out from your ISP what name they are using. This means that your mailbox is a subdirectory of your main home directory. This specifies the name of a file to use as a usage log. Procmail will write any diagnostic or error messages into this file during its activity.
Logs can be useful for tracking down errors when creating new new recipes.
After the recipe is proven to be working you don't need the log file. This defines the location of the sendmail program. This is required for forwarding and autoresponding to incoming email. This line is not required. It is a comment line used to remind us of what the heck the script is supposed to be accomplishing.