You can do both.
The card can be used to convert at the interbank exchange rate at the point of purchase.
At the same time, if you want to ‘lock in’ a certain rate (e.g. You hold £s and you think that, due to a deteriorating economic situation or/and a higher chance of a ‘hard’ Brexit happening, the £ will fall in value. And therefore you want to exchange to a different currency. You would in anticipation of this, exchange your £s for $s or €s, for example.), you can by exchanging at the interbank exchange rate at the time of exchange.