All assets have their useful life. Even the long-term ones. That's why their value is depreciated by your accountant.
In depreciation, costs of acquiring an asset are slowly dissolved into your business. It's mainly accounting exercise, but there are practical reasons to endure it.
It allows you to apportion costs of fixed assets to your current profits, therefore giving better view on profitability and sustainability of your business.
Not only that, but it also allows you to increase profit in the year of acquisition and reduce taxes in subsequent years.
There are four inputs in calculating depreciation of each asset: acquisition price, estimated useful life, salvage value (for how much can you sell the old, worn out and used asset after its useful life) and method of apportioning.
The methods are fairly simple, either you use time as your basis or real usage (mileage for cars, output items for manufacturing line). Both can be either linear or use some sort of accelerating or decelerating, based on how the asset loses value both to your business and on the market.
Estimated useful life and salvage value are places where there is most leeway for companies to choose and use their own methodology. For that reason they are often fixed for tax purposes, but for accounting (especially managerial accounting), they should be set as close to reality as possible.
Very often companies use their asset over the originally estimated useful life, which isn't in line with accounting giving the fair and accurate assessment of the company. On the other hand, it follows the precautionary principle of undervaluing assets.
Linear time depreciation is calculated as (acquisition price - salvage value)/useful lifetime for a year, proportionally allocating costs for every year. Basing the estimate on mileage would work the same, just substitute usage for lifetime.
In these days, accountants have software to manage depreciating all assets. That is important especially when methods for each asset category differ and when having to maintain both financial accounting and tax accounting records.
But do not worry, your accountant should have your back. If they don't, hire another one.