Short answer: Not really necessary, but good if you can.
Long answer:
A suit comes into the picture only after the major necessities are covered. A relative once said: "Anybody can make excuses; but for the person who really wants to attend an event, they will make sure they are there, no matter what". Although his quote seemed like words of wisdom long ago, one now realizes that it is a bit too harsh on people. When dealing with randomness and emotional beings, it isn't right to apply hard rules on anyone.
The first most important thing you can do for someone's wedding, is to attend it when you have been invited. Being there for that person.
The second most important thing is not something for you to consider. It's for the hosts and the other guests to consider. Judging others. I know of people who scrutinize every tiny bit of detail about how another person dresses and grooms themselves. It's the equivalent of burying your head in your smartphone screen during an event. When at a wedding, try to help out with whatever you can and make the day special for the families of the bride and groom. At other times, socialize with others and try making them feel special by letting them talk about what they like and encouraging them to speak about events in their life which they cherish. When a person has shown the courtesy of attending an event, it is your duty, whether you are the guest or the host, to make that other person feel comfortable and welcome. Remember that rituals, customs and formalities are just a conjuring that are not anywhere close to being worth as much as a person's sense of self worth and respect.
That being said, if a wedding invite specifically mentions casuals, wear casuals. Else, the best bet are formals. If you have a good suit, wear it. If you don't have one or it's at the dry cleaner's, then a well groomed appearance with a formal shirt, pant and shoes will do just fine.
Remember: Mutual respect and consideration are far more important than a suit.
