When you're in a business requiring you to use tools and that you are driving around in your company car that is for example a Renault, having Renault Trafic racking and shelving. Because it can help a lot in both making sure everything is where it needs to be as well as keeping you from spending a lot of time looking for the things you need for the current task you're on.
I don't know many companies that offers shelves and racks for vans but WorkSystem UK is a company I know of at least that sells Renault Trafic racking among other things. With shelves and racks, you also don't have to worry much about your tools moving around too much inside your van when you drive around, which is especially useful when you've got things that tend to break easily. Breaking tools over time is probably more expensive than buying a rack for your van honestly.
Shelves and racks
With Renault Trafic racking you have less to worry about and time to take on more jobs which I presume is quite ideal for most people. Even so, I think most can agree on that it's better to keep your tools sorted and in place rather than not, which is generally what van racking and shelves are for.
I'm honestly not sure what more reasons you need, especially when it pays itself over time and makes your job a tad bit easier to do. And if you've or your company bought an expensive van, why not spend a little bit more to get it properly equipped for your line of work anyway?