There are two most common method of Encoding in telecom BER and PER i.e. basic encoding rules and packet encoding rules. If you see the structure of BER its is basically TLV (TLIV sometime) or something similar which means TAG + Length + Value. So for each IE there is a overhead of TAG+Length which is 4 bytes in general and is a huge overhead.
This overhead is ok if interface is wire-line but on air each bit/byte is costly specially because of the cost and scarcity spectrum. You want to minimize all overhead on Air and maximize the use of data that is the reason why packet encoding is used at RRC/ S1AP(contains NAS message) etc.