Using the Price Categories
You can use the price categories in a variety of ways.
It's easy for a GM to say to you, "You can afford two extra moderately priced things at the start of the game." Then you can look on the list and pick two moderately priced items without worrying about their cost. Plus, this approach makes it clear that you get two items, not twenty inexpensive items (which might be a lot to carry) and not an expensive item (which might not be appropriate for a starting character). The categories make it easy to lump similar items together.
The GM can also say "You can have whatever inexpensive items you want, and don't worry about the cost." For later stages of the campaign, when the PCs have more wealth, followers, and so on, the GM can do this with moderate or even expensive items. This allows the group to skip over playing through a shopping trip to get supplies, and you don't have to track prices down to the last coin or credit.
You can use the categories to estimate the cost of a lower- or higher-quality item. If an average-quality shirt (like a T-shirt) is an inexpensive item, a nice shirt (like a button-down shirt for an office job) is a moderately priced item, and an exceptional shirt (fashionable and/or suitable for a fancy event) is an expensive item. If a typical broadsword is an expensive item, a rusted or low-quality broadsword is a moderately priced item (and likely to break after a few fights), and a jeweled sword is a very expensive item.
Finally, you can use the categories as shorthand when evaluating loot, dividing up the spoils among the PCs, and resolving other story-based occurrences that crop up in the game without dealing in the minutiae of exact prices. This is especially useful in high-powered games where your PCs are rich and powerful.