Well, mysterious girl, my time on wiki has taught me , firstly that
spousal maintenance can be payable for a long time ( in my case it was 28 years ) and it is virtually inevitable that the financial circumstances of the parties will alter during such a long period.
Hence you need some mechanism for variation if the circumstances change. That provision for maintenance comes in the shape of section 31 of the Matrimonial Causes Act 1973.
Another thing that experience teaches you is that spousal maintenance can be insecure. People can, and do, lose their jobs.
Sooner or later the payer will retire and will no longer be able to
compy with the original order.
So from the point of view of the recipient, it makes sense to have
some income on one's own account over and above the spousal maintenance.
Capitalisation will be ordered only if the payer can afford to pay it.