I've had a look in a few books but can only offer a few pointers that might be helpful.
During the C19th, Finland was governed as part of Russia, so in looking for where contemporary descriptions of the import of timber from Finland, we need to look for descriptions of the trade with Russia.
As far as I can see the main distinction seen from this end of the trade was between the timber coming out of the White Sea ports of Archangel and Onega in the north (which we could think of as just Russian) and that from the Baltic sea and Gulf of Bothnia in the west - where Russian (ie Finnish) wood is considered alongside timber from Norway and Sweden. Frustratingly, the descriptions I have found all seem to concentrate on the Archangel timber.
As for the sizes, the practice was to import as 'baulks' (ie the whole log, squared) planks, deals or battens. The difference between these last three was in the breadth. Sawing into 'scantlings' was done by the timber merchant at the English end of the trade, not by the exporter, though by the end of the century this practice ceased to apply, as more timber was sawn into scantlings before export. The word 'batten' has lost its timber trade meaning and is now used just to refer to wood of very small section, used to hang roofing tiles from, or as an intermediate layer for fixing to.
These pages may help unravel some of the complexities - they are from "Every Man His Own Mechanic" a very readable book from 1882 which sought to explain to the thrifty middle classes how they could do their own jobs without employing any pesky tradesman.
As for the uses of battens from the Baltic, this description is of the Swedish side of the Baltic from very late in the century and confirms that softwood from there was really the general purpose building timber of the time, so in general, battens would be used for anything where the size was right for economical re-sawing and the quality fitted with the class of work.
But if the nub of your question is what might make riven battens preferable to sawn, then I can only guess along with the others. I reckon that the most likely uses would be for low-quality work where economy was more important than straightness. Disposable uses for packaging would make sense and so would further splitting into laths for lath and plaster construction. The second half of the C19th was when Britain was going through a huge building boom, with millions of cheaply built terraced houses constructed. These all had lath and plaster for ceilings and internal partitions; the lime plaster was pushed through the gaps between the laths and will stay in place even after it has come unstuck from the wood. (I've never seen lath and plaster against a brick wall though - that's a modern technique suitable for big sheets of factory-made plasterboard.) They also had plenty of the lower grade timbers - I've seen ceiling joists with bark on the side, well below the nominal dimensions.