Tin Blocks have no practical use in Tekkit. These blocks are usually used for decorative purposes, giving a nice metal finish to your buildings. They can also be used to save storage space.
Storage Saving Application
If you have too much tin then you could use 9 tin ingots to create these blocks for storage purposes to save space in chests. This is useful as they can be converted back to tin ingots. It takes 576 tin ingots to create one full stack of tin blocks, therefore condensing from approximately 9 full stacks of tin ingots into only one stack of tin blocks. Having a 9:1 efficiency rating, this is the same for all other metal blocks, such as the Iron Block and Copper Block.