Unless you operator a taxi or limo service, or charge for delivery of your product your investment in your car will never generate an income stream for you. Your car is one giant expense on your P&L.
A CMC can generate an income stream if you use it properly.
If you rent a CMC, the income stream that it generates must offset the expense in order to generate more profit.
If you purchase a CMC, the machine becomes an asset, increasing the value of your business. (See other threads on The Grumble regarding the future value of a framing business.) The asset is then deprecated over some time period reducing your taxable income during that time period, increasing net profit after taxes (NPAT). (NPAT is money to take home or re-invest in your business.) At some point in the life of the machine it can be sold returning money to the company which can be invested in a new CMC or other business venture.
Both renting or purchasing should reduce direct labor involved in generating the end product for sale. Thus more labor can be redirected to other profit generating activities. More profit generating activities completed without increasing labor costs should yield increasing net profit. Notice that I did not say that a CMC would reduce the cost of manufacturing the original product yet the use of a CMC can increase your bottom line if it is applied properly.
I am not an accountant, just a small business owner, like the rest of you. Consult your own accountant and tax laws regarding all accounting topics mentioned here. CMC can be replaced with chop saw, point nailer, or any other piece of equipment in all of the statements above.
P.S. Why buy a new CMC when there are plenty of used ones available? I support them all.