if you create another wallet you can make your transaction more anonymous, and not have all the transaction linked to the only wallet you have, and anyway how can they know WHO was using that address?
Imagine you're a ransomware cracker. You've sent a ransomware malware to Joe, who was so untidy not to have air-gapped backups, and he paid 10 bitcoin to address A (one of yours). You did the same with Jack, who sent also 10 bitcoin to address B. These addresses are known, of course, because Jack and Joe mentioned them when they put up a complaint.
As long as you are the happy owner of A and B, the chain will not teach you much. The problem is, what are you going to do with A and B ? You can transfer A to C and B to D. And then C to E and D to F. Great. One can follow that on the chain.
Now, are you going to put F on an exchange ? Exchanges want to know identities, especially if you want to get fiat. Whenever a successor (a "colored coin") hits an exchange, it is interesting to find out who owns it, because that person has received coins from someone who has received coins from the cracker, and hence one has a real-world trace.
So it is interesting to go and ask some questions to whomever it was that put F on an exchange. In this case, questions to the cracker himself. Of course, that doesn't prove, in itself, that the cracker was the cracker, but at least, that he knows the cracker, or knows someone who knows the cracker. So it is very easy: the first question is: "sir, from whom did you get the coins in address D ?". It is going to be difficult to indicate just anybody, because that person will not have done any business with the cracker, or will never have owned those coins. "an anonymous donation for my contributions on bitcointalk" is probably not going to cut it. So even after having transacted with oneself many times, the coins remain colored, and will trigger a "real world contact" from the moment they hit an exchange.
The same is true for any official store accepting bitcoins. If you've sold shoes to the cracker, you will probably have some trace (say, a post address, an e-mail) of where you sent the shoes. You will easily show that to law enforcement asking you about how you got those coins if ever YOU got them, and then you went to an exchange.
So it is very difficult for the cracker to get rid of his coins and not induce a real-world contact that may lead to him.
On top of that, it gets more difficult. Suppose the cracker wants to buy something worth 15 bitcoin. He will have to COMBINE both outputs, (10 + 10) and get 5 bitcoin in a return address G. As such, the combination of E and F in a same transaction hints that E and F at that point, are still the cracker's possession (we know that A and B are his possession, and it would be strange that these two tracks get separated, just to be spend together again later...). And we also know that address G is the cracker's.