Without padding, how would you encode, for example, a message with just a single zero? To be more precise, how do you distinguish it from two zeroes and three zeroes?
Both for encoding and decoding the padding is not needed. Without ='s, you get a uniquely different base64 encoding for NULL, 2 NULLs and 3 NULLs.
This shows the binary, base64 without padding and base64 with padding:
NULL --> AA --> AA==
NULL NULL --> AAA --> AAA=
NULL NULL NULL --> AAAA --> AAAA
As you can see, all the padding does is make the base64 length a multiple of 4. You already get uniquely distinguishable symbols for the 3 cases (one, two or three NULL symbols) without the ='s, so they are unnecessary
The output padding is only relevant for decoding. For encoding, since the alphabet of Base64 is 6 bits wide, the padding is 0 when the input is not a multiple of 6 (e.g. encoding two bytes (16 bits) needs two more bits to become a multiple of 6 (18))
Refer to the "examples" section of the wikipedia page
