In theory, income taxes tax "wages", "salaries", and "net income".
The design of a tax system is an important political issue. A good tax system will raise the amount of revenue that the government "needs", and seem fair to both most of the people who vote, and to the people who pay most of the taxes. If the tax system does not seem fair to the voters, the legislature is likely to be thrown out of office. If the tax system does not seem fair to the tax payers, the government will have a difficult time collecting the taxes.
There are countries that tax dividends from businesses, but not the profits of businesses that stay reinvested in the business. There are countries that tax the profits of businesses, but don't "double-tax" the dividends from businesses. And there are countries that tax both the profits of businesses, and dividends from businesses. It sounds like you are in a country that thinks it is fair to tax business profits, but does not think it would be fair to pretend that the assets used up by a business are free.
Anyone designing a tax system needs to think about how someone could "game" the system. If it is possible for business owners to figure out how to not pay any taxes, some of them will. If none of the business owners pay any taxes, the voters will notice.
Suppose you could re-invest all of your profits in physical assets for your business, and those assets retained most of their value for a long time. (For an extreme case, imagine that you bought a chair made of gold, and most of the value of the chair was the gold.) If you could deduct the full cost of the purchase immediately, then you could avoid all taxes on your profits. This would make the tax system seem unfair to most voters (who earn wages and salaries, not business profits.)
Thus, most governments choose a compromise. By letting businesses "depreciate" physical assets over the assets over their estimated useful lives, the governments recognize that the decline in the value of the assets is a cost of doing business. This does not seem completely unfair to most businessmen. This compromise means that most business people wind up paying some taxes, which makes the system seem fair to people who are paying taxes on wages and salaries.
In theory, "depreciation" is supposed to approximately match the decline in value of an asset due to your business' use of it (or due to the time you hold it, because your business might need it). In theory, you only put up with the decline in value of the asset because the asset helps your business make money in other ways. Thus, the "depreciation schedule" is supposed to match up the cost (of the declining asset) with the revenue (at each time in the future) that it should help you earn. In theory, the total amount that is allocated adds up to the total decline in the value of the original asset. If you invest wisely, the incremental revenue you gain (because of the asset) should exceed the cost of the asset. In theory, the incremental revenue you gain per period (because of the asset) might be proportional to the depreciation per period.
In practice, there are lots of fudge factors. Many investments are a waste of money. Other investments are spectacularly good. Sometimes you can figure out which investments were good in hindsight. Some things wear out faster than expected, or need to be replaced for unrelated reasons. Other things are useful (and perhaps even valuable) indefinitely.