We often explain the Braille System by showing how Louis Braille arranged the alphabet into rows of ten. The first row only uses the top two rows of the cell and covers the letters a-j. The second row adds the three dot to a-j to create k-t. And the third row adds dots 3 and 6 to a-j to create the remaining letters. The symbol for “W” does not have the three dot, and is obviously out of order. We often say that there was no “W” in French and that the symbol for “W” was added later.
Note that there is a more interesting and correct answer to the question of why “W” does not follow the logic of Braille’s system. Actually, the “W” was there from the beginning, at least it is included in the first publication of the system in 1829. But it was in the Fourth Row of symbols. That row adds only the 6 dot to a-j and covers nine symbols common to French but not normally used in English, mostly accented vowels. At the end of the fourth row, basically a “J” with the 6 dot added, is the “W.” “W” appeared in French only in borrowed words from English and German, so Braille delegated it to a lower priority role. But it was not added later, it was just very uncommon in French, so was accorded a less important position in his table of symbols.