Why do we need to prefix module name with Julia function sometimes to make it work?

As example, here is a DateTime object declared below using Dates module:

julia> using Dates

julia> dt = DateTime(2021, 05, 11, 18, 21, 50)

We can find the Hour value of dt simply using hour() function from Dates module:

julia> hour(dt)

We can prefix the module name with hour() function also:

julia> Dates.hour(dt)

Now, if we try to use format() function from Dates module as below:

julia> format(dt, "e, H:M:S dd-u-yyyy")
ERROR: UndefVarError: format not defined

it throws error. We need to prefix module name here to make it work:

julia> Dates.format(dt, "e, H:M:S dd-u-yyyy")
"Tue, 18:21:50 11-May-2021"

Why do some functions in Julia need to be prefixed by the module name to work?

using Dates brings only the functions and types which were labeled with export inside the Dates module. For more about export and using, see: Modules · The Julia Language