In today's world the internet has opened up endless possibilities that didn't exist years ago. Homeowners can buy anything they want from a middle man (who purchases from the manufacturer). You can buy furniture, lighting, rugs, artwork, accessories.....pretty much anything you want from anywhere.
So why use a Designer???
One issue is size & scale. Most homeowners just don't know how to layout their space correctly and know/order the appropriate sizes. What do you do - when you ordered a sectional and it doesn't fit or you order a chandelier and it is way too small or so large you can't walk under it? When you employ a Designer, they will create a furniture / space plan for you to show you how everything lays out in your space and how it will fit. They understand the correct lighting sizes for the different areas in your home and what mounting heights work best for hanging them. Do you have stairs, freight elevator, etc...? A Designer also plans in advance to make sure all items will be able to be moved to the final location. They check elevator dimensions, stair sizes/corners, etc. making sure all can delivered & installed.
Another issue - colors. Finishes and Fabrics look differently on a computer screen. Designers NEVER order anything without having samples in hand to check and make sure all colors, tones, hues and tints are correct for the space and design. When you utilize a Designer they will be able to have all of the fabrics, swatches, finish samples in person to show you rather than viewing from a screen.
Another issue - quality & comfort. You can't tell quality from a computer screen. Designers KNOW the quality of the manufacturers they buy from and they buy directly from manufacturers. They visit the showrooms and factories so they understand the comfort level, materials and techniques involved with the fabrication. They can advise on which upholstery and even cushion types will be best for you based on your unique needs/wants.
Lastly - problems & damage. Who is going to handle if something goes wrong? The sofa is the wrong fabric, the delivery company broke the lamp or the leather recliner arrived with a major tear. Designers deal with freight and damage claims on a daily basis. They have a variety of sources and trades at their fingertips to fix / repair or replace a damaged item. They have established relationships with manufacturers. They handle all of these headaches so their customer/client doesn't have to.