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Spam prevention is important. Spammers can have a huge impact on your server before you even know they're there. When spammers hit the server en masse, it can bring a server to its knees. Why? Your server uses the resources it has in order to serve the website to visitors. In most cases, you probably have enough resources to serve your members just fine (unless you use shared hosting).
However, when a lot of spammers hit at once, they eat up the resources your server needs in order to show the site. That can cause the website dreaded 500 internal server error, cannot connect to the database, etc. When your server crashes, it can cause database corruption and other issues - which can result in loss of valuable data.
To prevent that, you need to either do some work yourself, or hire someone such as BryZar to do it for you, available in the top menu for Support.
One thing you can do yourself is use the free or paid Cloudflare or you can get BryZar to do it if you are not sure how to set it up. Cloudflare can prevent the spammers from even reaching the server. This is important as it keeps them from using up the resources. It is not fail-proof though and so it won't remove 100% of the spam but it does help a lot.
Another free tool is Project Honeypot and though part of that tutorial is for a specific script, the first part shows how to get a Honeypot. You can then put it on your site, whichever script you use. Project Honeypot can help find harvesters and other spammers and adds them to the blacklist. This can help with many spam fighting methods as the more spammers that are caught, the more the other tools that use blacklists can prevent them from getting on your site if you use them.
We use both Cloudflare and Honeypots as Cloudflare was made by Project Honeypot so they work well together.