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An asset used over several years is not an expense of the year it was bought. Depreciation spreads its cost across the periods that benefit from it, so profit is not distorted by the timing of purchases.
It is an allocation, not a valuation. The book value after depreciation is not what the asset would sell for.
Straight-line charges an equal amount each year: simple, predictable, and appropriate where an asset delivers value evenly.
Reducing balance charges a percentage of the remaining value, so more falls in the early years. It suits assets that lose value quickly at first — vehicles and technology being the standard examples.
The one reflecting how the asset delivers value. Straight-line for even use; reducing balance for front-loaded loss.
No, and they usually do not. The difference is reconciled in the tax computation.
Unrecorded disposals, and assets expensed rather than capitalised.
Half an hour with your own data usually saves reading three of these. The guides will still be here afterwards.