After altering those settings, you'll move on to manage the contact and lookup functions. When it comes to who can send friend requests on the site, the option is really a personal one. Most people (myself included) opt to leave this set to everyone. However, to lower the amount of network spam from random users, the strict filtering option is enabled for Inbox messages.
For lookups, friends only is the safest bet, otherwise anyone who already sees your email address or phone number, can use those to search for your profile. Also, if you allow search engines to link to your profile, then sites like Google and Bing can index and link to your profile and timeline.