In preparing an input for SHA256 or RIPEMD-160 a single bit (1) is appended to the message before adding any necessary zero padding and before adding the bitlength (64 bits) to form 1 or more 512 bit blocks. What is the purpose of adding the single bit (1)?
If you look at the Wikipedia on padding article page, you will see many different ideas.
The main problem when designing a padding, firstly, one has to consider how to eliminate the padding correctly, all the time. It is the designer's choice, as noted in and the original RFC 4634 -- SHA and HMAC-SHA.
Once you put 1 followed by many 000 to the end of a string, you can remove the padding by looking the first position of a single 1 from the end. That was the simplest idea for padding, AFAIK.
When considering that hashes only supply integrity, If finding a collusion is easy with this (or any) padding than the hash is already failed there.