Agencies tend to answer this question in their own favour. The honest answer is that a ready theme is the right choice more often than the industry likes to say, and the wrong choice in ways that are hard to see until later.
Where a theme is genuinely right
If the site exists so that someone who already heard your name can find you, see what you do and reach you, a theme does that job. Paying for a custom build to achieve it is spending money on something the visitor will never notice.
The same is true when the content fits an ordinary shape: a few services, some references, a contact page. Templates exist because that shape is common, and it is common because it works.
The bill for a ready theme is not issued when you buy it. It is issued the day it no longer fits your business.
The cost you do not see
A theme is built to cover as many cases as possible, which means carrying code for cases that are not yours. That weight shows up as slower pages, and page speed is now part of how you are ranked and how patient a visitor is.
The second cost is friction. Every time you need something the theme did not anticipate, you either give up on it or pay someone to fight the template. Those small fights add up quietly until a rebuild looks cheaper.
Three questions that decide it
How will success be measured? If the answer is that the site should simply exist, a theme is enough. If the answer is that enquiries should arrive, the layout has to be built for that, and templates rarely are.
Does the content fit a template? A service list and a contact page do. A multi level product structure, a dealer network or several languages do not, and forcing them in is where the friction starts.
Where is the business in two years? If it is doing the same thing at the same size, buy the theme. If a dealer network, another language or online orders are coming, build for that now rather than twice.
The path between
There is a middle option that gets overlooked. Use a proven platform for the parts that are solved problems, orders, stock, payment, and build the interface for your brand and your conversion path. You keep the reliability and pay only for what is genuinely yours.
If you want to work out which of the three fits your situation, we can go through it against your actual content and plans.