What is a carry-along yarn used for?

Dear Community,
When—or rather, for what—do you need a carry-along yarn?

There are at least two uses for a reinforcement thread.

1. Practical use:
To reinforce areas of a knitted garment that are subject to particularly heavy wear, such as the heel or toe of a sock. To do this, however, you need to know where the “weak spot” is. Some people wear through the heel but not the toe, while for others it’s the other way around.
  The elbow area on sweaters or jackets can also be subject to more wear than the rest of the garment. Here, too, it can make sense to reinforce it if you know that the wearer has already worn out previous garments in that area.

2. Decorative use:
The classic example would be a thin glitter thread if you want to add a bit more festive sparkle to a knitting or crochet project without buying special yarn for it. There are also thin yarns with sequins or tiny beads spun right into them. (Your go-to search engine can help you find something like this.) 
Even yarns that are actually intended for use on their own can be used as a companion yarn—for example, a thin mohair-silk blend combined with sock yarn to make a cozy sweater. 

A reinforcement thread is also very useful with alpaca wool, since alpaca wool isn’t spun as tightly. My mom once knitted me some alpaca socks made of pure alpaca—they were nice and warm, but they got holes in them in no time. 
 

Ricorumi makes a thin accent yarn that’s a bit “fuzzy”—it makes cotton amigurumi super fluffy in no time.

My favorite accent thread is Starlight by INLINIE
—it sparkles so beautifully 🤩☀️☀️

Sometimes, when the main yarn is too thin or could use a “splash of color or effect,” I add a carry-along yarn. It’s amazing how beautiful the result can be.

I’m always careful when using glitter yarn as a carry-along yarn so the effect doesn’t become too much. For example, I only knit the glitter carry-along thread along in every 4th round. 

Best regards, Ina

I bought some reflective carry-along yarn that reflects in the dark when it’s lit up by car headlights. I want to use this carry-along yarn to knit myself some warmers; that will increase my safety when I’m out in the dark in my wheelchair. 

Another reason to use carry-along yarn is that the toes of my socks get a lot of wear because of my crooked toe. Since I also knit very slowly for physical reasons
and it will take me forever, months in fact, to make a pair of socks, I’m going to knit the toes of my next socks with carry-along yarn. That way I can enjoy the long socks for a nice long time. 
The only thing I find a shame is that carry-along yarn often only comes in standard basic colors. It’s almost an extra hobby searching the internet for carry-along yarn in cheerful colors 😂
I’m really happy that I found orange, turquoise and purple a while ago. Because I have sock yarn in those colors here in my stash 

sorry for the long post 
Best wishes 
Kate