While it can be beneficial to take advantage of that extra space an attic offers, there are many factors to consider when you choose to use that area for storage or as a spare room.
For example, you need to make sure you properly insulate your attic to be liveable and ideal for storing items. Additionally, you need to check if the structure of your house can support the additional weight of the things you keep or of an extra living space level. You also need to consider the cost (it can be quite expensive, running from $5,000 and up) and the local building regulations in your locality regarding attic conversions.
Even if it requires a lot of work, having all that extra space for storage or as a living area can be well worth it. Just remember to place heavy importance on the cleaning and maintenance of your attic.