The main advantage of a ready-made company is speed — the company already exists, so you can act (sign a contract, open a bank account, bid in a tender) almost immediately. It is ideal when you need a company ID right away, or when you want a company with history or with VAT registration.
Founding a brand-new s.r.o. at a notary is usually cheaper today and, in a simple case, takes a matter of days. It makes sense if you are not in a hurry and want a tailor-made company from the start.
In short: ready-made = instant availability and the certainty of paid-up capital; a new s.r.o. = lower cost but a longer wait for registration. Choose according to what matters most to you.