What does the Catholic Church teach about Tithing:
Although the Church teaches that offering some form of material support to the Church is obligatory for all Catholic adults who are able to do so, it doesn't specify what percent of one's income should be given. Remember, tithing was an Old Testament obligation that was incumbent on the Jews under the Law of Moses. Christians are dispensed from the obligation of tithing (meaning giving 10% of your income) minimum percent of their incomes, but not from the obligation to help the Church.
The key to understanding how God wants us to give to the Church is found in 1 Corinthians 16:2, "On the first day of the week [Sunday] each of you should set aside whatever he can afford," and in 2 Corinthians 9:5-8.
To paraphrase: God doesn't demand a fixed amount of money from us; he wants us to give from the heart. If people are forced by their church to give a certain percent of their income, that's extortion. If they give freely and cheerfully the amount they are able, that's a gift.
Why is it important?
In order to fully fund the church, and at the same time the diocese, both entities have pledge drives in order to forecast. When a parishioner pledges, or promises to donate, a certain amount of money, the church is to forecast and plan on at a minimum that amount to budget. The pledge of a tithe allows both parishes to forecast and budget, in the same way, the diocese/Archdiocese is able to forecast based on their pledges and offerings received.
Information for this article gleaned from:
http://www.catholic.com/quickquestions/what-is-the-churchs-position-on-tithing
http://www.syracusediocese.org/giving-donations/diocesan-planned-giving-program/#
https://www.cathdal.org/Giving_Donations