Some altcoins are created for experimental purpose, to see how people react if this kind of innovation is implemented.  They created by cloning or forking chain thru modifying the codes, some starts from scratch but have the  other early coins source code as reference.
One of the obvious benefits of alternative coin is experiencing the transaction in more cheaper way and at the same time, it helps promotes bitcoin to people.

Other altcoins are competitors to bitcoin. And all of them play their role in all process of bitcoin standing. To me other altcoins are just trying to get the same level as bitcoin. And look - they are also growing in their price, so if you are haven't such money to invest couple thousands $ in bitcoin you may invest in one of other which cost cheap today, but who knows what the price will be in some years.

Altcoin devs say that their coin will compete bitcoin and defeat it but have you seen one that has had an actual success doing so? No. Some are increasing in price, some are decreasing, just the natural thing that happens in every market.

I wouldn't actually look for long term investment in most altcoins because the majority of them are there for the devs to make money. They abandon the project once they've made money and then make another ICO project. Rinse and repeat. That's what's happening. Most coins only stay alive for a month or so and then becomes stable. No noticeable price increase or price decrease going on.

The most common interaction of bitcoin with alts that I'm seeing is that people are bitcoin to buy alts. Sometimes they would use the profit the gained to buy more bitcoins, sometimes they just convert it to fiat but just stock it in their digital wallet and they'd use that to buy bitcoin again if they feel it's wise to do so. I'm not sure of the extent of interaction between them but I believe it's sizable enough to make some difference.

Bitcoin with few altcoins would have been great, that created an opportunity to trade in the market. But not where we are standing now. We have around 800 odd altcoins which is ruining the market actually. Investments are getting divided in to those 800 altcoins which could have been divided in to few altcoins and bitcoins. If this happened, the bitcoin price would have touched more than 2500 grand.

Nowadays if you have 100 USD to invest, you can make your own coin with the help of a programmer. This is not good and it is attracting the investments which should have been flowed in to bitcoin.

At my perspective Altcoins don't have any effect on Bitcoin. Every month there are new Altcoins introduced in ICOs and they come and go but I don't see that it affected Bitcoin in anyway. Bitcoin has the largest market share in terms of users/owners, also not all people are traders so you can see how it cannot affect the price outcome of Bitcoin.

Though bitcoin does not need any support as it can stand alone.. but for sure alt coins are an alternate support for it as the markets are running because people buy and sell alt coins in order to grow their bitcoins or earn some bitcoins... So alt coins are actually running because of bitcoin and alternatively they are a support for bitcoin.

At the time when the invested bitcoins remain in bitcoin then it do not affect the value of bitcoin but when for buying any altcoin the bitcoin users throw their bitcoins in the market then it affect a little the vale of bitcoin because by doing so the supply in the market increases. I think it happens only when they buy altcoins from exchanges while investing in ICOs do not have that affect on the value of bitcoin.

yes that is right as at the time when everyone was going to invest in ethereum then I saw that the price of bitcoin is reducing and the price of ethereum increase and after that when people once again were going to withdraw their money from ethereum to bitcoin then the value of bitcoin once again started to increase.