Since admission is free to the Lincoln Park Zoo, there are two main reasons to get a membership to the zoo: to save money on parking, and simply to support the zoo.
Details on those benefits, and a few smaller benefits are below. See More Information for a more detailed comparison of savings based on member level.
Parking
Every level of membership comes with discounted parking: individual members ($69) pay only $12 for parking on each visit, household members ($95) get two free parking passes, and then pay $12 for each visit after that, and safari members ($189) get free parking all year. A typical visit might cost a non-member $30-40 for parking, so an individual member will save money after just 3 visits.
Reciprocal Admission
Members of Lincoln Park Zoo receive free or discounted admission at many other zoos around the country. You can go to the
AZA's reciprocal admissions program page to get a full list of other zoos in the program. A zoo listed in that program will have one of three things listed in the "reciprocity" column. 50% means that Lincoln Park Zoo members receive 50% off that zoos admission cost. 100% & 50% means that an LP Zoo member will either receive half off or free admission at the zoo (depending on the policy of that zoo—it's their choice how to handle members of zoos that are themselves free). Free means that everyone receives free admission at that zoo.
There are a couple of important rules with the program: an individual member at the Lincoln Park Zoo will only receive a discount for one person. A household or above member will receive a discount for more, but the other zoo may have limits on how many discounted admissions they offer a household. You also may not receive free parking at other zoos. Lastly, zoos that are near Chicago may not offer discounts; most notably, Lincoln Park Zoo members don't get discounts at Shedd Aquarium or the the Milwaukee Zoo.
Other Membership Benefits
All members receive a 10% discount at the Park Place Cafe and the Patio at Cafe Brauer (the two best restaurants at the zoo), as well as gift shops. There is also a 50% discount on stroller rentals (saving $4.50 on single strollers or $6.50 for double strollers). Members also have access to the very nice Member Lounge by the east entrance. The Member Lounge has the best bathroom in the zoo, as well as a relaxing space to rest. There are also members-only events, including a members-only night at the very popular Zoo Lights and members-only mornings (an occasional early zoo opening just for members).
A full list of member benefits are available on the zoo's membership webpage.