Because of two reasons.
First, I believe in the coins which I'm supporting. Every coin which I have chosen, from Bitcoin to lesser known altcoins like FuturoCoin, has an interesting idea behind it, which I found worth of my support.
Second, I have invested only amount of money which I'm eventually ready to lose.

I'm optimistic about it. I believe that full adoption of crypto will eventually happen one day. But how fast it will occurs it only depends on us. Cryptocurrencies are already, without any doubts, very important part of our world. There are available as a payment method in more and more places. For many coins (like Dash, or based on its code FuturoCoin), facilitating transactions is the most important goal. So I think that everything goes in a right directions

It's always good time to at least consider an investment. The market is very variable and it's hard to say when is a good time to start getting into it. I think that the most important think is to do some research first. I think that current state is good because prices are still relatively low, but you have to check it by yourself, because "low" could be understand very different. You should also check news from developers of crypto which you want to support. Maybe it will be some updates about their current plans which will be interesting for you and will help you to decide (for example, FuturoCoin team has recently informed about masternode update, which was crucial news for my further suport of this coin: https://newsroom.futurocoin.com/become-a-masterode-owner/). If developer team is active that means the currency will still be improving.

This is the main reason why I investing in them. Crypto are fascinating technology which may change our lives, and I want to support this mostly because of that reason, not because of money which I can earn. So when I choose currency which I'll invest in, I'm looking mostly on their ideas for future. Good contact with community is also important, that's why I support coins which put much effort into this, like FuturoCoin or Dash.

I'm quite new in the subject of stablecoins, till this time I've sticked to BTC and altcoins. But when few days ago TrueUSD was added to CoinDeal, an exchange where I'm trading, then I started to consider this as an investment option. Any basic advises? and what are the differences between TrueUSD and USDT, cause I have only found here info about that second one

Of course. Diversification is good in every industry, especially in cryptocurrency, which is aimed for people who value freedom. I'm glad that new cryptocurrencies have chance to achieve success and they are competitive to those who are on the market from quite long time (like in case of FuturoCoin, which despite being only one year old currency has become the first one which is sponsoring one of Formula 1 teams: https://newsroom.futurocoin.com/new-currency-for-aston-martin-red-bull-racing/ )

As for me, I would point FuturoCoin as worth of attention. It’s vastly developing, after only one year it’s already present on 11 exchanges. In addition, this is the first crypto which is the sponsor of Formula 1 racing team.
But still, this is only an opinion of some guy from the internet, you should check their website: https://futurocoin.com
And do your research about other altcoins, who know which will be the most suitable to you.