The reason an LLC would want to take advantage of Subchapter S (and be taxed as a S Corp) is to minimize self-employment taxes (aka payroll taxes).
The self-employment tax rate is currently 15.3%. This rate consists of two parts: 12.4% for social security and 2.9% for Medicare. The owners of an LLC pay self-employment taxes (and income taxes) on the LLC's profits.
If you are (1) an owner of an LLC AND (2) an employee receiving W2 wages, you only pay self-employment taxes on your salary, if reasonable (not the LLC's profits).
For example, if a single-member LLC's profit for 2018 is $100,000, the owner will pay self-employment tax of $15,300 (15.3% of $100,000). If however, the owner pays himself a salary of $60,000, he will pay self-employment taxes of $9,180 (15.3% of $60,000).
As you can see, the ability to minimize self-employment tax can be a huge benefit year after year. Should you have questions about self-employment tax, including the implications of your entity decision and IRS elections, you should consult your tax professional.