Profit is simply all of a company's sales revenue and any other gains minus its expenses and any losses. A $3,000 depreciation expense, then, has the effect of reducing profit by $3,000. It's important to note, however, that "profit" is really just an accounting creation. With the truck in the previous example, your business spent the money upfront. All of the money was gone as soon as you bought the truck. But as far as your profit-and-loss calculations are concerned, you didn't really give up any value. Instead, you just traded $30,000 worth of cash for $30,000 worth of truck. As time passes and you "use up" that value by using the truck, you turn the cost into an expense through depreciation
Impact In Cashflow
If your business were accounting for the truck using cash accounting -- the method people use to balance their checkbooks -- you would have shown a $30,000 expense when you bought the truck and no expense at any time afterward. Under straight-line depreciation, you show no expense at the start, then $3,000 a year for 10 years. In each case, your overall profits decline by $30,000; it's just a matter of timing. The "real-world" effect is on cash flow. In both cases, you have a $30,000 outflow of cash at the beginning and no outflow afterward.
Impact in Tax Accounting
Though most companies use straight-line depreciation for their financial accounting, many use a different method for tax purposes. (This is perfectly legal and common.) When calculating their tax liability, they use an accelerated schedule that moves most of the depreciation to the earliest years of the asset's useful life. That produces a greater expense in those years, which means lower profits -- which, since businesses get taxed on their profits, means a lower tax bill in the earlier years.