Migration kettle 4.2 --> 6: MetaStore

Hi,

I'm currently migrating from kettle 4.2 to 6. Our code runs kettle transformation by instanciating TransMeta and Trans classes in java. I see now that TransMeta has a constructor that takes a IMetaStore object as parameter. I also see that this metaStore can be passed as null. What do I lose if I don't pass any metastore? If I want to pass one, which one should be used, based on what?
Thanks a lot!

Regards,
Nicolas