To allow you to still track the location of these products and have them participate in the reservation logic.

"Storable" means you can enable procurement logic (Odoo will watch the stock levels based on reordering rules and raise Purchase Orders).

"Expensing" a product just means you don't intend to tie it directly to the production of a good or service (so it will never be a "cost of goods").

Examples of storable products you might want to Expense could include low value (less than what would be considered a fixed asset) tools or cases that you need to track quantity and location of.

Imagine moving a Pallet Jack into a location and updating this in Odoo so you know where it is, having already expensed the cost of it.

Imagine needing to manage inventory or 10 tools or cases so you can reserve them for Manufacturing, having already expensed the cost of each.

If you have 10 lathes / silicone molds / spools for tape or wire / containers for holding refrigerated parts and use them in a BoM (including having them as a byproduct) then Odoo would only show 10 MO's as "ready" to produce and each tool / case would be unavailable during the manufacturing process. 

Other storable products might have larger values and become fixed assets - so they would have no value in inventory as current assets, but would be "converted" into fixed assets during the Billing process.

Hello Ray, thanks for the detailed answer. 

How we can expense out these types of storable products already enabled as the expense? I mean what is the procedure?