Faster block times lead to faster transaction confirmations. Why was the Monero block time changed from 1 to 2 minutes?
If Monero usage increases so will orphan blocks and chain-reorganization. Bitcoin forks with much shorter average block times (compared to the 10/min per block BTC average) have far more many orphans. CryptoNote based coins like Monero are more susceptible to the effect of chain reorganizations than Bitcoin forks so it was decided that 1 minute blocks were too short. Longer confirmation times make chain reorganizations less common.
Perhaps the strongest reason to make this change is to guard against an increase in popularity (and the size of the network) causing the chain to become unstable. This has been seen on Bitcoin forks with 30 second blocks, where long chain (10 blocks or more) reorgs become extremely common and the network may fail to converge at all for long periods of time (or in theory permanently). Similar effects are seen to a lesser extent on BTC forks with 60 second blocks, and CryptoNote is somewhat more sensitive to the effects of reorgs, so staying at 60 seconds puts us in (or at best close to) a potential danger zone. The coins with the fastest block times that don't seem to commonly run into this trouble are the LTC-style 2.5 minute blocks.
2 minute block times will also cause the Monero blockchain to grow in size at much a slower rate (particularly now when many blocks are still empty) thereby improving scalability.
The above benefits were balanced against the impact slower confirmation times may have on merchant and user convenience. In the future if network latency and/or transaction volume changes dramatically the block time decision can be revisited.
Current chain use is low, and many blocks are empty. Every empty block adds close to 300 bytes, and outputs from the block reward.
After some discussion, it was agreed that the block time could be increased from 1 to 2 minutes in order to diminish the impact of those, as well as decrease the number of orphans.
It is relevant to note that the 1 minute block time was originally chosen arbitrarily at a time where faster blocks were all the rage, and so there was no real arguments for 1 minute vs another target.