It is a really broad question, how long are the essays? (I didn't study A-levels).
I'd start by introducing enzymes as biological catalysts and how they function; induced fit, activation energy, accelerate reactions. You could follow this up by explaining that they're ubiquitous in virtually all metabolic pathways and biochemical processes. Then, critically, emphasise how these vital reactions would be unfeasible without enzymes and, hence, that makes them essential for all life.
In the main body I imagine the best approach would be to document several vital processes that are dependent on enzymes, and mention the specific enzymes involved. I guess the amount you write about depends on how many marks the essays worth. There's so many examples to choose from: Respiration and ATP production, Muscle Contraction, Digestion, Nerve Transmission (Na/K Pump), DNA replication, Transcription and Protein Synthesis.... If you want to venture away from animals, Photosynthesis in plants, Bacteria producing Beta-lactamases to breakdown penicillins.... The list goes on. You could probably add in some nice diagrams for some of these processes too.
In the conclusion, reiterate that all the processes you've mentioned are vital for cellular and organismal function, yet are completely reliant on the presence of enzymes to function correctly; yet again showcasing how they're vital for life.
To answer your digestion question, if we lacked digestive enzymes then we would be unable to catabolise the large, complex molecules in foods into smaller, simple sugars and nutrients that we can absorb and use. We can see this naturally in people suffering from exocrine pancreatic insufficiency (EPI). In this condition, the pancreas doesn't make enough digestive enzymes and it can result in severe malnutrition. Disaccharidases break down complex sugars into simple ones, such as glucose; Proteases break down proteins into amino acids; and, lipases hydrolyse fats into glycerides and fatty acids. In all of these cases, the starting molecule is useless to us since it's too large to process. Enzymes break these molecules down into smaller molecules which we can then use for various cellular processes (e.g., amino acids: protein synthesis, glucose: respiration and lipids: membrane integrity).