The option to include referral links in the campaign depends on the nature of your perk module, if it is a competition, you would benefit from users being able to refer the url link to friends to increase the reach of your campaign. Also by sharing the link with more friends, users increase their chances of winning.
* If you turn off referral links for competition, the text on "Share with friends" will simply not be shown.
If you are giving out coupons you might want to turn referral links off as it could dilute the quality of your intended audiences if they simply share it with anyone.
You can turn referral links on/off for URL link and freeform depending on what you are trying to achieve and if the purpose of the campaign is to raise awareness with a larger audience, or if it is more specific for targeted C-level clients or key customers.