+ is a terrible operator for formating.

You got:

    price = 18.8
    date = datetime.datetime.now()

Now you want to save in a file:

    "It's 18.80 and we are the 01/04/2016"

With +:

    num, dec = str(price).split('.')
    msg = ("It's "+ num + "." + dec.ljust(2, '0') + 
          " and we are the " + date.strftime("%m/%d/%Y") + "\n")
    f.write(msg)

It is so long and ugly we have to break it on several lines to respect PEP8.

And it's a pain to write, or even worst, to read.

With print():

    num, dec = str(price).split('.')
    print("It's", num, ".", dec.ljust(2, '0'), 
          " and we are the ", date.strftime("%m/%d/%Y")), file=f)   

A tiny bit better since we don't have to use that many concatenation tricks.

If we didn't have to format the float, print() would have converted it to string for us which is nice (with + you have to call str() all the time).

With "%":

    f.write("It's %.2f and we are the %s\n" % (price, date.strftime("%m/%d/%Y")))

Way better, but still hard to read.

With format():

    f.write("It's {:.2f} and we are the {:%m/%d/%Y}\n".format(price, date))

Now we are getting sometwhere...

With fstrings:

    f.write("It's {price:.2f} and we are the {date:%m/%d/%Y}\n")

fstrings have everything:

    - easier / faster to type than any other versions;
    - easier to read than any other versions;
    - as expressive as any other versions.

It's reduce work by making you think less, type faster, read faster (for you and your colleague) and it's easier to spot bugs.

The only place where you don't want fstring are for l10n.