If you have an in-the-money option, then most likely the option is worth more than the difference between the strike and current price.
In that case, it makes more sense to "sell to close" the option and to buy the stock at the current market price. You'll have more profit left over from the premium received than what you "lose" by buying at market versus the strike price.
For example, say the stock is currently at $110 and you hold a call option with a strike of $100, that is currently worth $12. If you exercise the option, you buy the stock at $100 and have an instant paper gain of $10 per share, minus what you initially paid for the option.
If instead you sell the option and buy the stock at $110, you still have a stock that's worth 110 but have $12 per share in cash, for a gain of $12.
The only significant difference might be tax, since you'd have to pay tax this year on any gain from the sale of the option ($12 less what you paid for it), versus deferring the gain on the stock until you sold it, with a cost basis of $100 plus what you paid for the option. Unless you got the option extremely cheaply and the current tax is significant, it's probably not worth foregoing the profit just to defer the tax.