I own a .com website. At what point should a site owner also consider buying up the rest of the domains (.net, .org, etc)? What are the advantages of this?
Redirecting is still beneficial, it allows people who may assume you use .net to redirect to the .com without question - that reduces the potential for misdirection.
If you’re in the EU I would buy the ones mentioned and for regional TLD’s buy .EU and the TLD for the country you’re in (like .ES / .FR / .UK or whatever). 
Hello Alex, been away for a while, but now I am back 
Even though you’re buying all the extensions up, would they just lay dormant there purely for somebody else not to register them or would you put something on them.
Firstly, you can’t buy them all up - many extensions require you to be a resident of the country before you can purchase that TLD legally. As for buying up domains, the main benefit is that you will control the TLD’s so you can prevent copycat sites (to a certain extent) and you can ensure no matter why extension they think is right, it’ll guide them to your official domain. On that note, I recommend buying up the main top level extensions (COM / NET / ORG + possibly BIZ / INFO / TEL) and your regional ccTLD (like US or CO.UK + ORG.UK, etc) to give you a solid starting point, hope that helps answer your question. 
Buy, or register? They are 2 diffrent ways.
I suppose if they are awailible for the registration it worth to get all of them and that will not be so expensive. I suppose you can consider that as investment.
Buy them up as you have a use for them.
You can just park them or redirect to your core domain name.
Buy them up as you have a use for them.
About the only one that might be worth buying just to protect your name is the .net and depending on where you live it may also be useful to get the one with your country specific TLD.
Advantage is that others cannot hack your business and trademark.
Makes sense, thanks. Personally I will not be putting separate content on each domain, will take forever to do and update, and I cannot really see a benefit in doing it.
Well it is better if you can set up some specific content on each one and then link them all together. The redirect option is only really useful if you bought the domain just to make sure no one else gets it and you don’t have any separate content to put on it at the moment.
So the only thing you would do is set up a redirect?
The main ones to be focussed I feel are the .net, countries T.L.D. (so .us if your from the USA), and .com if possible. There are so many other extensions. I live in Europe and a while back they released .EU domain, so I could probably register that too. But like you said there are literally hundreds of domain names. It won’t reasonable to register country domains unless you have a presence there.
It kind of depends on the name. Most people will naturally go to the .com of a name, but if you don’t want anyone to have the same website name, you could register them. It only costs you about $20 per year more to get the .net and .org
You can’t buy up all of the several hundred different extensions unless you have a business presence in every country but for those that you do buy where you don’t have something specific you want to use it for then parking it on top of content you already have and setting up a 301 redirect is probably the best option - that way you make your site independent of the TLD and if someone types example.net then they end up on example.com which is where you would prefer for them to be.
There isn’t much point in buying up domains if you are not going to point them to something of yours as the point of buying them is to prevent them being used by competitors and if you don’t point them to your site then they will just be advertising the registrar you bought them from - exactly the opposite of why you bought them in the first place).
It would be a different matter for someone investing in domains who intends selling them again where they would park them on an advertising service so as to earn money from them rather than parking them on their own content.