A network of (“thousands of”) servers has — like most things — pros and cons.
Some of the pros are:
The network is more resiliant against outages. If lemmy.ml is down, all other users can still access the network.
It's hard to take legal action against the network or to buy it out (like Big Players™ like to do to get rid of potential competitors).
It allows various similar or even conflicting moderation policies. The network, i.e. the infrastructure doesn't allow or prohibit any specific opinion (the communities do).
It allows for different ways to pay the bills: goodwill of the admin, donaitions, ads, fee or selfhosting. The latter also allows great control over the data so you control your privacy.
Some of the cons are:
Content is replicated across servers, which increases the total amount of data stored.
Latency and speed suffer.
Interoperability with the wider Fediverse is less than 100%, which can create confusion and frustration.