For a simplified example, let's assume that this year you bring in $100 of cash and spend $50 of it on opex and $50 of it on capex.

In this case, your revenue this year is $100, but your expenses this year are all $50 of opex plus some percentage (depends on over how many years you capitalize the investment, as set by your policies but limited by tax law) of capex, if it's a 5-year depreciation then it would be $10 of those opex $50, so for your $100 of outgoing cash $60 are expenses this year and $40 is treated as accumulated assets for future. So the accounting says that you have $40 of profit this year - and, crucially, it could have been as low as $0 or as high as $80 (this year! The total profit doesn't change, but it can get moved around in time to a different year) if your accountants can make the case that those activities were 100% opex or 100% capex.

And while there are various reasons why the company might prefer these numbers to look higher or lower, the main practical impacts are twofold - one is taxes, where saying that some expense is opex means you pay less taxes today, deferring them for a few years and improving your cash flow, but the other is stock market, where saying that some expense is capex means that your quarterly profit is higher and thus it helps your stock price and any employee benefits linked to stock price.

And because of that your management often will have KPIs with a very strong motivation to push things towards opex or capex depending on the company.