So you have a glass sliding door that is just over a metre wide. The average sized roller is about the size of a 20c coin. Now, every time you open and close the door, each roller travels 2 metres. Now, if you open and close the door, say... 10 times a day? 7 days a week? 52 weeks a year? over 5 years?...
The Burj-Khalifa building in Dubai is currently the worlds tallest building, standing at 828 metres, 160 stories high. (To give that some local scale, the Bunbury Tower is only 70 metres high).
Now if you were able to turn that building on its side, in 5 years your sliding door roller has travelled from the base of that building to the very tip, and back again, just under an incredible 44 times!
All on rollers slightly bigger than a 20c coin and you thought they didn't do much work!