just to say it was taught in Y1 at our school (yes mrz I know what your thinking - but for both DDs addition/ subtraction/ counting at intervals was KS1 business).
gymastics - counting at intervals of 2/5/10 is doing several things
as you said, it's laying the foundation for understanding x2, x5 and x10 tables.
but
it's teaching children standard intervals to jump ahead (preparation for addition in general and carrying in addition) - if you presume that counting standardly prepares them for adding 1 - if you also know intervals for 2/ 5/ 10 you basically can add more swiftly making a nubmer of mental jumps:
for example 8 + 7 can be broken down into 8 + 2 = 10/ 10 + 2 = 12/ 12 + 2 = 14 and 14 + 1 = 15 or 8 + 5 = 13 and 13 + 2 = 15.
It's not exactly a straightforward or obvious jump from the concept of counting in intervals to using that knowledge to assist more complex addition - but it's starting to build that underlying understanding of how numbers work.
in some schools they also work this backwards - so counting down from 100 in intervals of 10 - teaching them standard intervals for borrowing (in preparation for subtraction involving borrowing).
it's also teaching them patterns.
all mutliples of 10 end in zero
all multiples of 5 end in 5 or zero
all multiples of 2 end 0/2/4/6/8
this pattern becomes useful in decoding factors (what x what makes 60 - that ends in zero so factors can include 2, 5 or 10).
finally - learning to count by twos also teaches doubling/ halving - both of which are fundamental concepts underpinning a lot of later maths work - including learning other times tables (i.e. x4/ x6/ x8 and x12 - which can be learned as doubling other times table facts you know - i.e. 4 x 8 is the same thing as 2 x 8 = 16 and then double that = 32).
they may also notice that 5 + 5 = 10 (2 fives = 1 ten) and start to learn that relationship - anything x 5 is half of what it would be x10.
HTH