Why use size 5 DMC Pearl Cotton thread inside of size 10 crochet thread?
Both are close in gauge when crocheting.
Some of the newer versions of crochet thread are as soft as the DMC, though not all.
The answer is simple: color variety.
With crochet thread you can find one maybe two colors of blue, pink, green and yellow, but rarely at the same store. They usually have white, black, red, green, pink, purple and maybe yellow or recently I found orange.
DMC almost always have five or so skeins in verying hues of the same color. Si if you want to make a necklace to match a favorite dress or shirt you're more likely to find it using the DMC Pearl Cotton than in the crochet thread.
DMC is found in the needlework-cross stitch-embroidery isles in most craft stores. The handging ones are ususally the size 5 thread with size 8 and 12 in balls underneath. They are the twisted skeins usually hanging on a peg board. You will need to untwist the thread and roll it into a ball so it doesn't tangle when you try to use it, but each 27 yard skein makes a ball about the size of a quarter and is fairly quick to untwist and roll.
That brings me to the other reason I like the DMC thread: the 27 yard skeins.
Crochet thread is usually found in a 250-350 yard range, which is great if you are making something large but over kill if you're just making a necklace.
Sure crochet thread will work, but there are more color variety with DMC Pearl Cotton.
P.S. If any one is wondering why I no long put the color numbers I use for my designs samples it is because I want to incourage you to go out and look at all the colors out there and then pick the one that you like best.