What is the purpose of JSON encoding a table and decoding it I thought roblox did that for us?

You can't save a string with a size larger than 260,000 characters for a single key, so one may want to check the size of their JSON encoded table before saving it. For example:

local t = {}

for i = 1, 2^14 do
    t[i] = true
end

local json_t = HttpService:JSONEncode(t)

if #json_t > 260000 then
    -- handle error
else
    data:SetAsync("test", t)
end

There is no other major reason that you should care about encoding your datum to JSON before saving it unless you need to check something before the save occurs.