It matters because the conveyer belt systems used at some airports have limits on the individual baggage weight; which is why it is enforced on an item level, and is stated plainly so at Etihad's website:
Maximum weight and dimensions
- To / from Brazil, Canada and the US: 158cm (50x70x38)
- To / from all other destinations: 207cm (45x72x90)
Each item must not exceed 32kg. This applies to all baggage including
pre-purchased excess baggage. Baggage that exceeds the limits and
dimensions shall only be processed through Etihad Cargo.
So we established the maximum each individual item can weigh is 32 KG no matter what class you are traveling on.
Now we have to see what is the included allowance - what you are allowed to carry for free on each class. So for economy this is 2 bags, each of which has a free allowance of 23 KG.
Note that on some routes, only a total is given; and not by piece. This means you can check any number of bags, as long as each individual bag is not more than 32 KG. If you exceed the allowance for your class, you'll have to pay extra.
If your itinerary is restricted on both item and weight; and you wanted to check one bag that exceed 23 KG in economy, you have a few choices:
If the weight is more than 23, but less than 32 of your individual bag, then just pay the excess weight fee.
If the weight of the bag is more than 32 KG, and you have already checked in one bag - then you'll have to pay to check in an extra piece of luggage. Once you fill in the additional luggage, and its weight is more than 23 KG (your free allowance), you'll have to pay an additional fee to bump the allowed weight to the maximum of 32 KG.
Repeat the above, till your luggage is split into either 23 KG pieces, or 32 KG pieces.
On routes that don't define a piece limit but instead list the total weight - you will only pay for the excess weight.
How much you'll pay, depends on the routes - and is available online at the excess baggage fee website.
If you feel you'll be needing excess baggage allowances, it is better to pay online rather than at the counter as there is usually a discount for online payment.