Disclaimer: I know almost nothing about this business. I do think the following could be a good explanation:
If removing a bolt takes 2 minutes and moving from corner to corner (at height, carrying a power tool capable of unbolting the bolts) another 2, you can gain 2x8+3x2=22 minutes per section removed by removing the bolts before the dismantling crane is attached to the part. Multiply by 6 or so on a day, and you can go home two hours earlier (or pay all personnel involved for two fewer hours), and (possibly the most important factor) rent that heavy-lifting crane for two fewer hours.
Also, you probably are right that the heavy-lift crane takes more time to do its work than removing the bolts. If so, the workers who loosen the bolts are mostly waiting, but they are still being paid.
If removing a fully tightened bolt takes more than one person (say because the tool used to do so is big and heavy; these bolts probably are inches in diameter), while removing a loosened one can be done by one, it also pays to have two persons loosen all bolts in the morning, send one home (or to another job) and only pay one in the afternoon to remove the loosened bolts.
If removing a bolt takes 2 minutes and moving from corner to corner (at height, carrying a power tool capable of unbolting the bolts) another 2, you can gain 2x8+3x2=22 minutes per section removed by removing the bolts before the dismantling crane is attached to the part. Multiply by 6 or so on a day, and you can go home two hours earlier (or pay all personnel involved for two fewer hours), and (possibly the most important factor) rent that heavy-lifting crane for two fewer hours.
Also, you probably are right that the heavy-lift crane takes more time to do its work than removing the bolts. If so, the workers who loosen the bolts are mostly waiting, but they are still being paid.
If removing a fully tightened bolt takes more than one person (say because the tool used to do so is big and heavy; these bolts probably are inches in diameter), while removing a loosened one can be done by one, it also pays to have two persons loosen all bolts in the morning, send one home (or to another job) and only pay one in the afternoon to remove the loosened bolts.