My understanding is that consumption = consumption of final goods = whatever consumers buy from companies (with an adjustment for export-import-process + maybe housing is slightly different (not important here). So if you were to buy a fully US-made table it wouldn't really matter that first one company plants and harvests the tree, another then buys the whole tree and transforms it into useable wood (cuts into useable sizes, sorts out bad parts, possibly treats wood etc), and then the last step somebody buys that wood and assembles a table. The only thing that matters is when this table is actually sold to an private customer (for 100\$ or whatever)(assuming its directly sold by the joiner and leaving out dedicated retailer).
So far seems somewhat understandable that only the use of the activity to real persons/consumers would ultimately be relevant and seen as economic productivity (use being measured by $spent). However I don't understand why some expenditures made that are called "investments" still increase GDP (and aren't lumped in with other precursors as non-final goods)?(1)
(1)(Maybe expenditures that are classified as investments, like a new production machine, can be seen as not directly tied to a specific output-product (unlike e.g. a concrete amount of wood bought per output-table to be assembled into a table)? But i would not see why this is directly relevant when measuring the quality/productivity of the whole economy as GDP probably should (just because somebody invests into something doesn't mean that's reasonable, e.g. investing into gold-plated machines etc or just machines not fit for their intended use, while you could somewhat reasonably argue that everythng bought by customers should be defined as a useful product and GDP-increasing because apparently the customer found it useful and satisfying customers is ultimately the purpose of the economy?
Also where would that leave sandpaper and similar things? (Machines detoriating very little with each table produced, wood deterioirating/disappearing completely (into the table), and sandpaper in between depending a bit on luck with the wood and not clear whether its more non-final good or investment))