Fully Associative cache offset bits
It's necessary because, as the text says, you copy an entire page in the cache when a specific value is required. A page consists in more bytes (32 if I'm correct), and accessing the single location in the cache line requires an address (the offset).

So, in the fully associative architecture, the block in the RAM can be mapped in each block of the cache, but still the single byte has to be addressed by the offset.