@william blake,
a typical use-case for business laptops is in combination with a docking station and a bigger external monitor. 17 inch laptops are too large for most people to carry them around, even if it is just in the office. Thats why 14 inch and 15.6 inch variants are much more popular than 17 inch laptops.
Reliability just means that business laptops are typically put through more quality and longevity tests. These systems are supposed to last at least three years and if they break during their warranty (which can be extended to up to 5 years in some cases), the repairs are expensive for the manufacturers. Thats why business laptops tend to be more reliably compared to cheap consumer PCs that you may buy in a Wallmart or Best Buy, since these PCs are mainly optimized to offer the most "bang for a buck", but not really made to last long. Consumer laptops often have just a 1 or 2 year warranty as well.