What do all of those designations after an agent's name really mean and why should I pay attention anyhow?

Those designations indicate that a real estate professionals has received additional training and has certain expertise, knowledge and training in specialized areas that require basic real estate skills.  If you real estate transaction requires an understanding of 1031 exchange for example, you'll want to work specifically with an agent who has been trained and is proficient in handling these types of transactions.  Consider to the same as you would as if you were going to a medial specialists.  If you had a brain disorder, you would seek the services of a neurologist and so on and so forth.  Not every real estate professional has these real estate designations, so those who extend their real estate knowledge are more knowledgeable and also have more experience. Here is  list of designations and business expertise:

The GRI
The Graduate, REALTOR® Institute (known as the GRI) designation is bestowed by the California Association of REALTORS®. It certifies that the individual has completed an intensive series of 14 lecture courses in areas such as: marketing and negotiating skills, technology, new disclosures and the legal aspects of real estate, and the latest trends in financing. The GRI program is considered by many real estate industry professionals to be the most advanced and comprehensive training program available to real estate agents in North America.

The CRB
The Certified Real Estate Brokerage Manager (CRB) designation, awarded by the Council of Real Estate Brokerage Managers, is recognized throughout the industry as the measure of success for real estate brokerage management. By completing the rigorous program requirements, brokers master the skills and knowledge necessary to confront the ever-changing industry challenges, and run their businesses more productively and profitably. The CRB designation distinguishes managing brokers who have achieved an advanced level of education, experience and expertise in the real estate field. Only 3% of all REALTORS® have achieved the prestigious CRB designation.

The CRS
The coveted Certified Residential Specialist (CRS) designation, awarded by the Council of Residential Specialists, is nationally recognized and the highest professional designation awarded to REALTORS® in the residential sales field. The CRS designation recognizes professional accomplishments in both experience and education. Less than 4% of all REALTORS® hold the CRS designation.

The SRES
The Seniors Real Estate Specialist (SRES) designation certifies that the holder has received training specific to the real estate needs of adults over 55 years of age. Courses cover such topics as: how to help seniors make wise decisions about selling the family home, buying rental property, or managing capital gains and tax implications of dealing in real estate. The SRES designation is granted by the Senior Advantage Real Estate Council—an organization consisting of real estate agents who specialize in meeting the unique needs and concerns of maturing individuals.

e-PRO
The e-PRO designation is the online internet training program specifically designed to certify real estate professionals as “internet professionals”. Comprehensive and interactive, e-PRO is the only certification of its kind recognized nationwide and endorsed by the NATIONAL ASSOCIATION OF REALTORS®.