To start, Planter Bags are ideal if you are very limited for space. The fact that they are 'flexible' bags means that you can squeeze them into tight spaces, where a standard, rigid planter may not fit. And if you've got very little storage space, planter bags are a better choice than rigid planters - you can empty out the planter bag, wash them clean and store them flat until needed again, so taking up very little storage space.
Planter Bags are suitable for raising a large range of crops as they are available in different depths - the depth of the planter becomes important when you consider the root length of some crops - carrots, parsnips, beetroot etc. need a deep planter to grow in for a successful crop. Whilst lettuce, radishes, spring onions etc. can be grown in more shallow containers without affecting cropping.
Planter Bags don't just have to be used to raise crops outdoors - they are also ideal for use in greenhouses or polytunnels. Their compact size means you can fit them in easily and use them to raise such crops as cucumber, peppers, tomatoes etc which can benefit from the protection of a greenhouse at the start of the season and can be moved outdoors once the warmer weather arrives.